Bootstrapping an existing environment with no .jenv file no longer destr...

https://bugs.launchpad.net/juju-core/+bug/1336843

Tests are not yet written as there will be much overlap with another PR which is yet to land. I will not merge this without tests.
These warnings could be reworded as a user does not know what "prepare" means. Does a PrepareFromName failure mean that the jenv file is missing? It should re-create it if it's missing. I'm a little unclear how the failure described in the bug ie removing a jenv file causing an error is triggered? Is it because the PrepareFromName fails if an existing env exists? Let's discuss live.