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.
Scared me for a second as I had no recollection of modifying this! It looks like this was a result of a merge from trunk by voidspace. Possibly it's since been removed causing this diff. I'll merge trunk again which should take care of this.