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 comment was supposed to be in the context of this method. In that context, isn't this the two ways we get an environment?

In my local tests (using a local provider) it states that the state port is already in use. Is there a better way to check to see if it's already bootstrapped than letting the prepare fail?