engine: Host with no display network should not be selected to run VMs

If ConfigValue.OnlyRequiredNetworksMandatoryForVdsSelection is false,
or if the cluster's display network is not attached to the host, the host
shouldn't be selected/eligible to run VMs in that cluster.

Change-Id: Ie90e2821f2299be85b1d4c9a6f064a4053f93e73
Bug-Url: https://bugzilla.redhat.com/949434
Signed-off-by: Alona Kaplan <alkaplan@redhat.com>