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: I893d88a9e65f8675a7983f3accfdfde45a1982f5

os-cloud/openstack-ansible-galera_server,os-cloud/openstack-ansible-galera_server,openstack/openstack-ansible-galera_server,openstack/openstack-ansible-galera_server