salt-cloud GCE : Support for addresses & internal IPs

Here is a small patch that enables all the GCE Address manipulation function from salt-cloud.
Also allows for creating VMs and Load Balancers with named fixed IP addresses inside GCE.

The patch also includes a cloud provider parameter "ssh_interface" that specifies if the hosts will use their private IPs or public IPs for salt communication. (Just like the EC2 module does.)

Also followed in with a docs update.

Hopefully this is helpful!

-b