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: I56978c553775407d57f22bbeeef219f30de0225f

openstack/openstack-ansible-rsyslog_client,openstack/openstack-ansible-rsyslog_client