Remove --master-netdev default value

Since the --master-netdev option is now used both at cluster init and at
cluster modify time, it cannot have a default value of DEFAULT_BRIDGE
(xen-br0) anymore. As such at cluster init we make the initialization
explicit if another value hasn't been passed. This fixes gnt-cluster
modify changing the master_netdev to xen-br0 if called without that
argument.

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