GPFS volume encryption-at-rest support

GPFS volume encryption-at-rest support

This patch extends the capability of gpfs driver to support volume encryption
at rest using GPFS Native Encryption feature (https://ibm.biz/BdXPBm).

This includes:
1. Verification of configured gpfs backend to support encryption at rest.
2. Driver exposes gpfs_encryption_rest capability via volume type convention.

Usage - Cloud admin creates a volume type "encrypted" with extra-specs
  gpfs_encryption_rest=True
Every volume created using type "encrypted", the volumes will be encrypted
at rest.

DocImpact

Needs an update in gpfs driver documentation, as this patch introduces
encrypted backend support.

Change-Id: I83949ca51668834ecbce52e90527cf06f95bd001
Implements: blueprint gpfs-volume-encryption-rest