Removed the SecurityGroupExtension

This removes the SecurityGroupExtension from Google Compute Engine.

The current implementation does match what is expected from that extension. In GCE, firewall rules are applied to a particular network, and there is no notion of *global* security groups.

The current implementation maps a security group to a network, and then applies the rules to that network, but that is not appropriate. The number of networks that can be created is limited, and managing the security groups could end up exhausting the networks, making it not possible to create new instances.

The way GCE works don't fit in the security group extension, and it should be better removed. A FirewallExtesion or similar would make sense if there are other providers that work in a similar way.

/cc @danbroudy @andreaturli