Support for different environments files

You can currently switch between environments.yaml files by putting them in different dirs (production, beta, alpha) and setting $JUJU_HOME to the appropriate value per command.

It's not appropriate to set a different source for environments.yaml: juju stores important environment info (like the CA cert), by environment name, next to the environments.yaml file in which it is defined, and the risk of catastrophe caused by using two such files in the same dir seems to outweigh the inconvenience of specifying the environment variable.

But, hmm; would something like `--home envs/production` be useful to you? If so, that'd be very simple.

--home would indeed help qa team which is working with multiple homes.

The Juju home directory is required by the juju tool during preparation for command line processing creating a chicken-and-egg problem: --home can't be used until command line processing is done and command line processing can't be done without knowing the Juju home directory.

After discussing with axc and fwereade we've decided that this ticket isn't going to get done.

The easiest way around this is to just do:

    JUJU_HOME=some/path juju ...