Admin user created from server side base_data.xml files,

<record model="res.users" id="base.user_root">
            <field name="partner_id" ref="base.partner_root"/>
            <field name="company_id" ref="main_company"/>
            <field name="company_ids" eval="[(4, ref('main_company'))]"/>
            <field name="menu_id" ref="action_menu_admin"/>
            <field name="signature">-- Administrator</field>
        </record>