core: extract PrepareSnapshotConfig method to ImagesHandler

Extracted PrepareSnapshotConfigWithoutImageSingleImage
from BaseImagesCommand into ImagesHandler for reusability
in other commands.

Change-Id: I8c5e8d2763df3bcf18fb13a030148de352f57846
Bug-Url: https://bugzilla.redhat.com/1056935
Signed-off-by: Daniel Erez <derez@redhat.com>