Fix volume_compatible_with_resource_definition?

If the new resource does not specify a snapshot_id, the check to see if
we have an existing volume will always fail if the existing volume had a
snapshot made elsewhere. The fix is to bypass the volume snapshot
comparison if the new_resource snapshot_id is nil.
Merge pull request #18 from streeter/snapshot-check

COOK-2659: Fix volume_compatible_with_resource_definition?