New 1.21 branch, master is now 1.22

[Martin Packman]

Following the 1.21-alpha3 release, a 1.21 branch has been created for
stabilisation, and the next release from master will be 1.22-alpha1.

Please continue to do new feature work against master, but be aware
that some bug fixes may also need backporting.

The current list of work intended to go into the upcoming 1.21 release
can be seen on the launchpad milestone:

<https://launchpad.net/juju-core/+milestone/1.21-beta1>

If you have something else that should be in 1.21, please target the
bug against the 1.21-beta1 milestone. Any unfinished work will get
bumped to the next beta milestone on release.

Martin