Switch from oslosphinx to openstackdocstheme
Switch from oslosphinx to openstackdocstheme

As part of the docs migration work[0] for Pike we need to switch to use the
openstackdocstheme.

[0]https://review.openstack.org/#/c/472275/

Change-Id: I9e165a63b0e80a5773b3f9a3aa1d155d50070f6d

openstack/openstack-ansible-repo_server,openstack/openstack-ansible-repo_server