Use oslo_config.sphinxext to automatically document the available
configuration options in the sphinx documentation.
See https://etherpad.openstack.org/p/automating-oslo-config-documentation
for work to implement this to each project. Glance is one of the first
six projects for a consolidated operators configuration reference.
Change-Id: If701f7dfa240075457e6e149bcb12ff713a8c79b