GPFS Driver missing clone depth limit for snapshots

GPFS Driver missing clone depth limit for snapshots

GPFS driver is fixed to handle snapshot clones correctly. Previously,
these were allowed to grow without respecting limit defined in config
flag gpfs_max_clone_depth.  This change adds the depth check operation
in create_snapshot.

To ensure that all clone files are cleaned up, the delete_snapshot
method now marks snapshots to be deleted with ts file extension
and attempts to delete the snapshot.  If the snapshot cannot be
deleted because it has clone children, it will be deleted when the
child is deleted.

Closes-Bug: #1227366

Change-Id: I4fb2a720b55dbe033159e6fb341f6e2f1508776e