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.
I moved the check for whether or not the environment exists into the closure.

I'm not sure I understand your other point: why would we want to remove the jenv file if the prepare succeeds and the environment exists? Wouldn't that mean we created a jenv file where one didn't exist, and this is a good thing?