engine: Save gateway for host's active NIC

The gateway should be saved not only for the NIC that has the management
network, but also for the active NIC which the host communicated through
so that we can use it for example when the management network is not
provisioned on the host and we want to add it, such as when adding a new
host.

Change-Id: Ic4664e44880943b1b97559ef10f66aa82efad2fd
Bug-Url: https://bugzilla.redhat.com/972615
Signed-off-by: Mike Kolesnik <mkolesni@redhat.com>