charm should serve only releases (or make a release from a branch)

[Gary Poster]

Now that bug 1083618 is complete and bug 1083545 is almost complete, we should switch the charm to actually using releases. Moreover, in order to reduce charm breakages and charm test breakages caused by changes in the GUI, we should use releases exclusively.

I'm happy for this to be resolved other ways, but the following steps are what I envision need to be done.

- Make sure we have a release of the GUI--ideally both a stable and trunk version. See docs/process.rst to see the steps to follow.
- Make it so that the charm only serves releases. You should be able to choose either the stable or trunk series. Alternatively, if the charm user wants to use a branch, the charm should make a release from the branch and use it as the release that it serves. IOW, we never serve from a branch, but we might make a release from a branch to approximate this functionality.
- Change the charm tests to test with stable releases. The only use of the GUI branch should be to show that it can check out a branch, make a release, and install the release.
- Change the release information in docs/process.rst to specify that the charm should be run with the branch that is going to be released to make sure that it works. Ideally, this would be charm tests + qa, but I'm ok with this being qa only initially.

[Gary Poster (gary)]

Also, this will probably started after the branch for bug 1086794 lands. If so, getting the branch and making a release should use the NO_BZR environment variable (assuming that approach makes it through review) when running the Makefile target to make a release.