Proposed API Change - Get default repo/namespace from environment variable.

[Kapil Thangavelu]

Excerpts from Gustavo Niemeyer's message of 2012-02-28 18:32:01 -0500:
> > I think we've discussed this before, but how does one infer the series
> > without the default-series in environments.yaml?
> 
> Indeed, we did: https://bugs.launchpad.net/juju/+bug/886364
> 
> The reasonable choices for bootstrapping an environment are either:
> 
> a) The last LTS
> b) The Ubuntu running the juju command
> 
> After that, every other command should default to the series that
> originally bootstrapped the environment.

or c) constraints passed on cli to bootstrap.

cheers,

Kapil