breaking change: repository structure, deploy interface

[Clint Byrum]

Excerpts from William Reade's message of Thu Sep 29 03:48:57 -0700 2011:
> On Wed, 2011-09-28 at 23:31 -0700, Clint Byrum wrote:
> > This default selection based on the local machine series seems
> > totally arbitrary.  While we hope the world runs Ubuntu, the reality
> > is that many of our users will be using juju from OS X, or maybe even
> > Fedora/CentOS/RHEL... FreeBSD maybe?
> > 
> > I think we can safely set the default series to just 'oneiric' and when
> > the next series is "released" in the charm repository, bump the default
> > in juju to that, noting the update. Making this so dynamic will, I think,
> > harm repeatability of deployments.
> 
> I'm not sure that bumping the default-series will actually be any less
> harmful to repeatability. I guess it will probably happen less often,
> but I think that repeatability would demand that we make it a required
> key.
> 

Fair point, I have definitely overstated the impact to repeatability. I'd
think that users of the same version of juju with the same charms should
be able to expect the same behavior from deployed machines, regardless
of their client platform.

> What if we made it required, and changed the default environments.yaml
> (that gets generated if it doesn't already exist) to include a
> default-series of "oneiric"? We can update that as and when we please
> without changing anyone's existing environment.
> 

I think its fine to have it not required and predictably default to the
juju specified version. I see this as the path of least surprise.