core: extract PrepareSnapshotConfig method to BaseImagesCommand

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

Change-Id: I965a099ce8d048eb0b19bda96b387140b26bdf2b
Signed-off-by: Daniel Erez <derez@redhat.com>