juju switch doesn't work without an environments.yaml file

[Tim Penhey]

In the situation of someone being emailed a server file and them logging in, they may well not have an environments.yaml file.

[Cheryl Jennings (cherylj)]

A couple comments / questions around how all this should work:

I was playing around more with the multi-user / multi-environment experience and thinking about how we would modify switch to work without an environments.yaml file and realized that we could get naming collisions if we tried to have switch -l display a union of what's in the cache and what's in environments.yaml.

If we have an environment (or system) that isn't in the cache (or has a jenv file), but has a definition in environments.yaml, we could login to a server and give it the same name as something already defined in environments.yaml. If we allow this, how would we display this?

Also, would we want switch -l to display only environments that the user has in their cache / environments.yaml? Or would we want it to also show environments on systems we have access to, but haven't used yet?

When displaying hosted environments, should we use some sort of tree format to indicate which belong to which system?

[Cheryl Jennings (cherylj)]

Review request: http://reviews.vapour.ws/r/2657/

This PR just fixes switch to work with environments.yaml. Enhancements to list system / environments structure are not included.