NSX: Validate gateway device list against DB

This patch adds a check for validating the devices for a network
gateway are defined in the neutron DB before the request for
creating the network gateway is sent to the NSX backend.

Change-Id: Ic2245f965da332dbaee4ab5a869ce47f06a9e6ce
Partial-Bug: #1319581
Merge "NSX: Validate gateway device list against DB"