REST API extension to return IP Margin

[Aihua Edward Li]

It is desirable to have an API to return the number of available IP-address for each network. This information can used to assist nova to better schedule instance creation on networks with enough margin. It can also be used by monitoring tool to provide capacity monitoring, resulting better resource planning.

The proposed API is GET on /v2.0/<tenant-id>, the returned data is in the form of

{ "network-1-uuid": { "total-ips" : 256
                       "available-ips" : count1,
                       "name" : test-network,
                    }

}