This introduces a feature support matrix to illustrate which of our
various token providers supports which API operations and features. This
is intended to mirror Nova's feature support matrix documentation page,
found here:
http://docs.openstack.org/developer/nova/support-matrix.html
After running `tox -e docs`, the result is embedded in
`doc/build/html/configuration.html`.
Change-Id: I3dc896a2906e25827a9e01afc7de5a737831c336
Dolph Mathews