Fix net-gateway-create help message

"neutron help net-gateway-create" said:

  --device DEVICE Device info for this gateway device_id=<deviceidentifier>...

However, actually parameter name of DEVICE is "id". Not "device_id".

Change-Id: Ifa19f11f250101ddf4803c3c591bb19274fae869
Closes-Bug: #1262527
Merge "Fix net-gateway-create help message"