Only send gateway_ip on neutron subnet update when it's changed

Even if the gateway_ip is unchanged, neutron retruns error code
when the client trys to update gateway_ip which is already assigned.
In this fix, we will check gateway_ip is changed or not, then we send
the gateway_ip when it is changed.

Fixes bug 1227268
Change-Id: Ie6330a45a7998ffeb6a925be5bcc2c58ff9e6239
Merge "Only send gateway_ip on neutron subnet update when it's changed"