ceph/rbd: has a pretty strange error handling - install custom run_command function

Always use a custom error sub to get the real errors out of rbd command instead of the typical:

 2014-02-06 11:20:20.187190 7f3b6c37c760 -1 librbd: removing snapshot from header failed: (16) Device or resource busy

before:
rbd: snapshot 'abc' is protected from removal.
TASK ERROR: rbd snapshot vm-173-disk-1' error: 2014-02-06 11:06:02.438336 7f6f4ac92760 -1 librbd: removing snapshot from header failed: (16) Device or resource busy

now:
TASK ERROR: rbd: snapshot 'abc' is protected from removal.

Signed-off-by: Stefan Priebe <s.priebe@profihost.ag>
coding style cleanup
fix ' escaping of $snap parameter

Signed-off-by: Stefan Priebe <s.priebe@profihost.ag>
Storage Plugins: extend clone_image by snap parameter and add support to RBDPlugin

Signed-off-by: Stefan Priebe <s.priebe@profihost.ag>
cleanups
pass snapname to vdisk_clone and plugin clone_image

Signed-off-by: Stefan Priebe <s.priebe@profihost.ag>
rbd: unprotect snapshots before deletion

Signed-off-by: Stefan Priebe <s.priebe@profihost.ag>
RBD: enable clone of snaps

Signed-off-by: Stefan Priebe <s.priebe@profihost.ag>
bump version to 3.0-21