Make oslosphinx optional
Make oslosphinx optional

Avoid cyclic dependencies between pbr and oslosphinx. So if oslosphinx is not
available, continue to be able to generate the documentation.

Change-Id: I4c1f8ea5cded268388dab29931055223f8999c8a

openstack-dev/pbr,openstack-dev/pbr,openstack-dev/pbr