I got the same issue in Odoo 9 for handling custom fonts in RML reports. I have solved this problem by giving full font path in openerp/report/render/rml2pdf/customfonts.py. In the third element of tuple in CustomTTFonts in customfonts.py, rather than just to pass the name of the custom font use full path.
THSarabun', "THSarabun", "/usr/share/fonts/truetype/THSarabun/THSarabun.ttf", 'normal')..
e,g
CustomTTFonts = [ ('Sun-ExtA',"Sun-ExtA", "/usr/share/fonts/truetype/Sun-ExtA.ttf", 'normal'), ('THSarabun', "THSarabun", "/usr/share/fonts/truetype/THSarabun/THSarabun.ttf", 'normal'), ('THSarabun Bold', "THSarabun Bold", "/usr/share/fonts/truetype/THSarabun/THSarabun_Bold.ttf", 'bold'), ('THSarabun Italic', "THSarabun Italic", "/usr/share/fonts/truetype/THSarabun/THSarabun_Italic.ttf", 'italic'), ('THSarabun BoldItalic', "THSarabun BoldItalic", "/usr/share/fonts/truetype/THSarabun/THSarabun_BoldItalic.ttf", 'bolditalic'),]
Hope this will work.