VPNaaS: Cisco fix validation for GW IP

The validation to check that the router has a GW IP
for the Cisco service driver was trying to directly
get the gw_port information, which is not available, as
the validator only has a dict of the vpn_service info.

Modified validator to get the router object (not a
dict representation), using the vpn_service's router_id
and then check the attributes for the router directly.

Change-Id: I48286cda23d9737ddd90251dc0d1db1c310e7784
Closes-Bug: 1356127
Merge "VPNaaS: Cisco fix validation for GW IP"