Use openstackdocstheme for docs and release notes

Replace the usage of oslosphinx with openstackdocstheme and follow the
porting guide[1].

[1]
https://specs.openstack.org/openstack/docs-specs/specs/pike/os-manuals-migration.html

Change-Id: I8199b787c05406ba3997fdfe26d2bfcd31b329b4
3 files changed