This patch adds the support to set the gateway information
for a router.
Implements: blueprint neutron-client-advanced-router
Partially-Implements: blueprint network-commands-options
Change-Id: Ifb5a4d1965cd7e75c0c8cf2cfb677e0628b699dc
Depends-On: I2bda0dd40afd64b6cecca5f64ef2326bda4fac92