webadmin: Only display gateway if feature supported

Set the visibility of the gateway field, when editing a non-management
host network, according to whether the multiple gateway feature is
supported, so it wouldn't show in older clusters.

Change-Id: Ic31614a9a86e68a0840331f63ec2ac711ab8ac5f
Bug-Url: https://bugzilla.redhat.com/1008999
Signed-off-by: Lior Vernia <lvernia@redhat.com>