engine: Display network must have boot protocol

The scheduler should not select a host which the
display network is configured on without a proper
settings for IP address (i.e. only DHCP or Static
boot protocol are allowed).

Change-Id: I14e5a0edae8524a7334609e58934880a722ca87f
Bug-Url: https://bugzilla.redhat.com/955429
Signed-off-by: Moti Asayag <masayag@redhat.com>