core: Extract method of snapshot id.

Extracting a new method to get the snapshot id instead of
working directly with the parameters command.

Signed-off-by: Maor Lipchuk <mlipchuk@redhat.com>
Bug-Url: https://bugzilla.redhat.com/867339
Change-Id: I4754e381d70f08a9650596a4eeb3de296b3cb44f