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.
The above comments are not quite right. A jenv file is the result of preparing an environment using the info contained in a yaml file. I'd drop the comments entirely and add comments inside the if{}