restapi: Don't mandate a network when adding a template NIC

In template context, it should be possible to add a NIC even
without the network id/name since the network unlinking feature was
pushed.

The backend will know to fail the action if an empty network is sent and
the feature is unsupported.

Change-Id: Id5b1953178f4ff5e04a71688d8ccec5d3c789395
Bug-Url: https://bugzilla.redhat.com/952647
Signed-off-by: Mike Kolesnik <mkolesni@redhat.com>