Merge gating for more juju subprojects

[Martin Packman]

I've put up a gating job for goose on our jenkins slave for juju:

<http://juju-ci.vapour.ws/job/github-merge-goose/>

It will likely need some more work, but I fake-landed Katherine's
proposed branch and it passed. The switch over steps are disabling
direct landing for most/all contributers and getting everyone using
$$merge$$ as with juju.

Many of the other juju subprojects are also suitable for sending
through automated unit tests before landing, and I can easily make a
whole bunch more jobs for whichever packages we want to gate landing
on.

There are a few caveats:
* There isn't proper isolation with lxc yet, so test suites that do
dodgy things are still an issue.
* Most packages don't use dependencies.tsv so the deps are tied to the
merge job currently.
* Anything that needs a large external dependency like mongo isn't
catered for at the moment.

That said, if there are any packages you think we should *not* gate
landings on their test suite passing yet, please say now.

Thanks!

Martin