Libcloud 605 gce httplb

@erjohnso This is the PR I plan to send upstream, do you want to take a look first?

This also implements a change to create_forwarding_rule we discussed on the list, but does not yet update GCEForwardingRule similarly.
Good catch, thanks.  I've added these, though honestly if extra isn't passed to the constructor there's going to be KeyErrors all over the place anyway.  This isn't specific to this change and it's something we can address later if necessary.