Use new encryption provider constants
Use new encryption provider constants

These constants of 'luks' and 'plain' were introduced to Nova and
os-brick during Ocata in the following changes :

Ic155bd29d46059832cce970bf60375e7e472eca6

The direct use of the encryptor classes using their implementation class
path or name was deprecated in Ocata and will be blocked by
os-brick in Queens via the following change :

I3eec91e221692177833909e0378116cea4966807

Change-Id: Id221414d74af8413084c7935b762f93b7ce43c42

cisco-openstack/tempest,cisco-openstack/tempest,openstack/tempest,masayukig/tempest,masayukig/tempest,openstack/tempest