engine: Use collection for object names

For nameable objects, there's no need to use a list for creating a set
of the object names, just use any collection.

Change-Id: I9451323cf5b9a3edd5f538ada69d8427a7bd4145
Related-To: https://bugzilla.redhat.com/1055161
Signed-off-by: Mike Kolesnik <mkolesni@redhat.com>