Bootstrap no longer cleans up if it can't tell whether an environment is bootstrapped or not.

Added a test to simulate an instance in which this might occur. Tested locally; some tests were failing, but they looked innocuous and related to connecting to mongo, not these changes.
It is my understanding that this issue is caused by two instances of juju attempting to bootstrap the same environment at the same time, thus causing the race condition and one instance of juju to destroy the work of another instance of juju. I think this bug is caused by a deferred cleanup regardless of whether we are in an indeterminate state or not. If this is the case, I don't think we would want to remove the jenv file since this is exactly what we're trying to prevent. Clarification definitely welcome if I am resting on false assertions.

One idea that kept coming to mind is an OS semaphore to prevent this command from even executing if one is already running, but I was trying to keep the change small.