In  Odoo version 8 Select database / Manage Databases view using the below URL:-

http://localhost:8069/web/database/selector

Note: Default Port No 8069. change Url based on your port no configuration.

EDIT:

GitHub Source Code  https://github.com/prakashsukraj/Odoo-DBRestrict

I just finished a new module "web_dbrestrict" that Database Manager page restrict redirect to password page.

__openerp__.py

{
    'name': 'Web DB Restrict',
    'category': 'Hidden',
    'version': '1.0',
    'description': """
OpenERP Web core module.
========================
This module provides Database Manager page restrict redirect to password page.
        """,
    'depends': ['web'],
    'data': [     
        'views/web_dbrestrict.xml',        
    ],
    'installable': True,
    'application': True,
}

views/web_dbrestrict.xml

<?xml version="1.0" encoding="utf-8"?>
<openerp>
    <data>

    <template id="dbmanager_password" name="DB Password">
            <t t-call="web.login_layout">            
                <form  class="oe_login_form"  role="form" action="/web/dbmanager_password" method="post">
                    <div class="form-group field-password">
                        <label for="password" class="control-label">Password</label>
                        <input type="password" name="password" id="password" class="form-control" required="required" t-att-autofocus="'autofocus' if login else None"/>
                    </div>
                    <p class="alert alert-danger" t-if="error">
                        <t t-esc="error"/>
                    </p>
                    <p class="alert alert-success" t-if="message">
                        <t t-esc="message"/>
                    </p>
                    <div class="clearfix oe_login_buttons">
                        <button type="submit" class="btn btn-primary">Log in</button>
                    </div>
                </form>
            </t>
        </template>

    </data>
</openerp>

web_dbrestrict/controllers/main.py

import jinja2
import os
import simplejson
import sys
import openerp
import openerp.modules.registry
from openerp.tools import topological_sort
from openerp import http
from openerp.http import request, serialize_exception as _serialize_exception

if hasattr(sys, 'frozen'):
    # When running on compiled windows binary, we don't have access to package loader.
    path = os.path.realpath(os.path.join(os.path.dirname(__file__), '..', 'views'))
    loader = jinja2.FileSystemLoader(path)
else:
    loader = jinja2.PackageLoader('openerp.addons.web', "views")

env = jinja2.Environment(loader=loader, autoescape=True)
env.filters["json"] = simplejson.dumps

db_monodb = http.db_monodb

def module_installed_bypass_session(dbname):
    loadable = http.addons_manifest.keys()
    modules = {}
    try:
        registry = openerp.modules.registry.RegistryManager.get(dbname)
        with registry.cursor() as cr:
            m = registry.get('ir.module.module')
            # TODO The following code should move to ir.module.module.list_installed_modules()
            domain = [('state','=','installed'), ('name','in', loadable)]
            ids = m.search(cr, 1, [('state','=','installed'), ('name','in', loadable)])
            for module in m.read(cr, 1, ids, ['name', 'dependencies_id']):
                modules[module['name']] = []
                deps = module.get('dependencies_id')
                if deps:
                    deps_read = registry.get('ir.module.module.dependency').read(cr, 1, deps, ['name'])
                    dependencies = [i['name'] for i in deps_read]
                    modules[module['name']] = dependencies
    except Exception,e:
        pass
    sorted_modules = topological_sort(modules)
    return sorted_modules
    
def module_boot(db=None):
    server_wide_modules = openerp.conf.server_wide_modules or ['web']
    serverside = []
    dbside = []
    for i in server_wide_modules:
        if i in http.addons_manifest:
            serverside.append(i)
    monodb = db or db_monodb()
    if monodb:
        dbside = module_installed_bypass_session(monodb)
        dbside = [i for i in dbside if i not in serverside]
    addons = serverside + dbside
    return addons
 # In the above code Import and method copy and paste from the web/main.py file  
class Database_Password(openerp.addons.web.controllers.main.Database):

    @http.route('/web/database/manager', type='http', auth="none")
    def manager(self, **kw):
        request.session.logout()
        return http.local_redirect('/web/password')

    @http.route('/web/password', type='http', auth='public', website=True)
    def pasword(self, redirect=None, **post):    
        return request.render('web_dbrestrict.dbmanager_password', {'url_root': request.httprequest.url_root})
                
    @http.route('/web/dbmanager_password', type='http', auth='public', website=True)
    def dbmanager_password(self, cert_type=None, **post):
        password = request.params['password']    
        if password == 'Your_Password': #user enter password matched redirect to DBManager page
            return env.get_template("database_manager.html").render({
                'modules': simplejson.dumps(module_boot()),
            })    
        else:
            values = request.params.copy()        
            values['error'] = "Wrong password"                
            return request.render('web_dbrestrict.dbmanager_password', values)        
        
  web_dbrestrict/views

  copy and paste the "database_manager.html" File from the location web/views to  web_dbrestrict/views