Ensure cinder backend variables are defined

Ensure that variables like $e_backend are always defined even when the
backend is not enabled.

Will get rid of this error:

Error: Could not retrieve catalog from remote server: Error 400 on
SERVER: Failed to parse inline template: Could not find value for
'e_backend' at line 1 at
/usr/share/astapor/puppet/modules/quickstack/manifests/cinder_controller.pp:188
on node r64-acontrol.example.org