Optimize SecurityGroupsOutputController by len(servers)

The following patch adds an optimization for _extend_servers() so that
it calls get_instance_security_groups() when len(servers) == 1, and
get_instances_security_groups_bindings() when > 1. In addition, this patch
adds a return statement if not len(servers) as there is no reason to
perform a query if there are no servers.

Fixes bug 1182730

Change-Id: I149bf32176d60e72d76576168d15995f7215b657
Merge "Optimize SecurityGroupsOutputController by len(servers)"