Maintenance on oslosphinx ended with the Pike release. This
patch includes the following changes:
* switch to openstackdocstheme
* set context options for openstackdocstheme
- glance-specs doesn't have its own bug tracker, so config
to create bugs in the glance bug tracker with tag 'glance-specs'
* update requirements file
* update setup.cfg to use warning-is-error
* update four specs whose warnings were previously ignored
* add an '_extra' directory with stub .htaccess file
* configure docs build to include '_extra'
* update build command in tox.ini
Change-Id: I4da477fd39685a22fb4ef9d702af9b9a79bda0c2