core: filling blank external_id values

The following fills blank external_ids by creating content based on the entity ids.

Change-Id: I6d789381ff9e6022479cca1618f8d2a7accc9e71
Bug-Url: https://bugzilla.redhat.com/1091380
Signed-off-by: Yair Zaslavsky <yzaslavs@redhat.com>