juju switch

[Tim Penhey]

Hi People,

Jorge has been very determined to push for something like the bzr switch
command for working with multiple juju environments.

I've managed to work out how to implement `juju switch` without
violating the current behaviour.

Most of the developers just say "he should just use the JUJU_ENV
environment variable", as that takes precedence over the default
specified in environments.yaml.

We don't want switch modifying the main environments file, so what I
thought we could do is have something like:
  ${JUJU_HOME}/current-environment
as a text file that has a single line like:
  amazon

This file is then inserted into the precedence rules for looking for the
environment:
  1) ${JUJU_ENV}
  2) ${JUJU_HOME}/current-environment
  3) default from ${JUJU_HOME}/environments.yaml

`juju switch` could then just update that file.

juju switch some-env
  would check to make sure some-env is a valid environment, and specify
that in the file if it is, and error if not

juju switch --delete
  could delete the current-environment file to get back to "default"
behaviour


This is pretty trivial to do, and from comments from Jorge and Mark, I
think it adds actual value to the users. I think it is one day of work,
all tests included.

To make this more useful, I think `juju status` should output the
environment as the first line.

Cheers,
Tim