Update gnt-instance batch-create for NIC params

This is compatible with the previous version, but also allows specifying
more than one nic, by giving a "nics" list of dicts. The two methods
(individual fields for the first nic, and list of all nics) are
incompatible with each other.

The default remains one nick with no parameters specified.

Signed-off-by: Guido Trotter <ultrotter@google.com>
Reviewed-by: Iustin Pop <iustin@google.com>