engine: ConcurrentModificationException failing domain/host fail process

The following bugs were caused because of trying to iterate over collection and in
the same time (inside an iteration) try to modify it.
In order to solve a bug I removed one of the problematic collections, and move all logic to work
with one collection

Change-Id: Ie4e401263a6839370c7fccbf77b05d866fc06145
Bug-Url: https://bugzilla.redhat.com/958095
Bug-Url: https://bugzilla.redhat.com/958186
Signed-off-by: Michael Kublin <mkublin@redhat.com>