PPAs under stable release scheme

[Martin Pool]

At the moment we have three PPAs:

 * https://edge.launchpad.net/~bzr/+archive/ppa -- stable releases
 * https://edge.launchpad.net/~bzr-beta-ppa/+archive/ppa -- which
actually primarily means "release candidates" at the moment
 * https://edge.launchpad.net/~bzr-nightly-ppa/+archive/ppa -- nightly
from trunk

Under the new release process
<http://doc.bazaar-vcs.org/bzr.dev/developers/cycle.html> we could
potentially identify a few different channels:

 * nightly builds, but from the stable branch (currently 2.0) - this
would presumably be easy to add by just running another instance of
the robot
 * betas from trunk -- this probably makes sense in more than just
name to put into the ~bzr-beta-ppa
 * release candidates of the stable builds -- this could be a separate
archive, or they could go in the beta ppa, or they could go into the
main stable PPA

Since we set up this structure, Launchpad has gained the ability for a
single team to have more than one archive, so we can use that if we
want more PPAs rather than creating more teams.  I don't think it's
worth changing what already exists and breaking the URLs.

In practice I don't think any change is really necessary beyond maybe
nightly builds from the stable branch, and clarifying the
documentation.  During the period we're doing rcs for a stable release
there are unlikely to be any new betas, and we should get good testing
by putting them into the beta ppa.

-- 
Martin <http://launchpad.net/~mbp/>