core: add snapshots names to merge_snapshots exection message

* Added source/destination snapshots' descriptions into
  MERGE_SNAPSHOT execution message.
* Modified RemoveSnapshotSingleDiskCommand -> getJobMessageProperties
  accordingly.

Change-Id: I5933904ef5cf67502607091df270009b20ac6df2
Signed-off-by: Daniel Erez <derez@redhat.com>