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.
Can also be a `GCETargetInstance` (see #393) , just to make this even messier. TargetInstances are Zone based too. :/