core: extract SnapshotsEngineLock to BaseImagesCommand

Extracted SnapshotsEngineLock from  RemoveImageCommand
into BaseImagesCommand for reusability in other derivative commands.

Change-Id: Idc4b46a2c53a0ec6ac13c1dba73089ae4512bea7
Signed-off-by: Daniel Erez <derez@redhat.com>