Add support for storing volumes on GPFS

Key functions include efficient creation of snapshots and clones using
file system level copy-on-write mechanism. Copying of image blocks is
also avoided in case Glance backend repository happens to be on GPFS.

Change-Id: I0a532fe06798d07f21d79ee45444bb9c0a025430
Implements: blueprint gpfs-volume-driver
Merge "Add support for storing volumes on GPFS"