Replacing NetworkGateway when devices updated

Updating OS::Neutron::NetworkGateway failed when devices property changed.

Currently, NetworkGateway's devices property doesn't have update_allowed=True.
So, UpdateReplace have used when updating devices property.
However, Neutron doesn't allow that same Gateway Services ID exist at a time.
Eventually, 409 error occured by Neutron.

For avoiding above error, I implemented replacing NetworkGateway in handle_update.
The processing order is delete -> create. It can avoid the 409 error.

Change-Id: I4021610355d03eb6ca07272cb1e44ff2b31dfff9
Closes-bug: #1291901
Merge "Replacing NetworkGateway when devices updated"