engine: Network interface name is mandatory

The network interface name is used to identify the nic,
therefore it cannot be omitted. The patch adds a NotNull
constraint for the interface name.

Change-Id: I26f33bf12e9b53c3e31719b93a689e11e974acca
Bug-Url: https://bugzilla.redhat.com/979751
Signed-off-by: Moti Asayag <masayag@redhat.com>