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.
Nate and I came up with this name because we thought it was descriptive. Func is tautological, and only serves to differentiate the name. ProductionFunc at least describes its intent.