engine: select a host even if scheduling scoring failed

If no internal scoring function is selected it can result
In no host selected at all, even if some hosts did pass the
filtering stage.

Select the first host if none has returned from the scoring functions.

Change-Id: I96082ccdfd8759619e98bc437fdb468b3b583992
Bug-Url: https://bugzilla.redhat.com/1001986
Signed-off-by: Noam Slomianko <nslomian@redhat.com>