Allways set key_manager/api_class to an existing class path
Allways set key_manager/api_class to an existing class path

https://review.openstack.org/#/c/505752 has switched to use
key_manager.backend which is set by default to 'barbican'.
However, until fix in cinder [1] is merged, cinder expects
it to be a full class path. Castellan uses key_manager/api_class
to resolve key_manager.backend so, setting this parameter
to an existing class will fix jobs until [1] is merged.

[1] https://review.openstack.org/#/c/468252/

Change-Id: If25905cf4826b7e14d9699b70e2f524c12d1d813

openstack/puppet-openstack-integration,openstack/puppet-openstack-integration,openstack/puppet-openstack-integration