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.
There are two (+) variations of this:
 - Two concurrent attempts to bootstrap the same environment from the same $JUJU_HOME.
 - Two concurrent attempts to bootstrap the same environment from different machines/${JUJU_HOME}s.

The former scenario should be easily fixed. We could do a file-lock if it's worthwhile. In general, I find file locks to be more resilient to failure than things like process-shared semaphores; not sure if you're actually talking about implementation specifics here, though.

The latter scenario is the one I was thinking about. It's possible that on two separate machines that one could attempt to bootstrap the same environment. Both could create the .jenv file locally to then find (by inspecting remote storage) that the environment is already partially or fully bootstrapped. In *this* case, I think the .jenv file should be removed.