BZ#1305022 [RFE][cinder] Support volume encryption on NFS backends

Description of problem:

Hi
We've been testing cinder and nova volume encryption as detailed on the manual at https://access.redhat.com/documentation/en-US/Red_Hat_Enterprise_Linux_OpenStack_Platform/6/html-single/Administration_Guide/index.html#volume_advanced_encrypt

But it was not working properly on NFS backend and no information is provided there.

After some investigation it has been found that there's ongoing discussion on this http://osdir.com/ml/openstack-dev/2015-11/msg01907.html.

How reproducible:

Configure OSP cinder/nova encryption and validate as per http://docs.openstack.org/juno/config-reference/content/section_testing_encryption.html

Actual results:
If backend is NFS, the resulting data is visible, if it's dm volumes, it's encrypted

Expected results:
The data should be encrypted whatever the backend is

Additional info: