Add subnets attribute to OS::Neutron::Port resource

Added subnets attribute to the port resource to fetch the subnet
attributes such as gateway_ip which are associated with the port.

Change-Id: Ia809c343d5b7d1488b505f9e9e20da02b36de07d
Closes-Bug: #1336656
Merge "Add subnets attribute to OS::Neutron::Port resource"