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 also has dinged me in the past for not using named args. Likely OK here since there is plenty of digression and this is the only arg.

self.ex_get_healthcheck(name=healthcheck)