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.
# Kami gave this feedback to me on my last PR too, so just passing it along.

self.extra = extra or {}