Deprecate the keymgr_api_class input parameter and replace with new
keymgr_backend option. This mimics an equivalent set of changes in
puppet-cinder and puppet-nova, which were driven by a similar change
in Castellan's key manager options.
The deprecated value is still supported for backward compatibility.
Closes-Bug: #1732998
Change-Id: I5036c8de9a429f22e1828b4e37735f4aa47bd858