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 think this misses the case where:
1. prepare succeeds ie env != nil
2. environment exists
In the above case we still want to remove the jenv file (but not destroy the environment). We would need to add a test also.

Also, can we move the  "envExisted = ensureNotBootstrapped(env) == environs.ErrAlreadyBootstrapped" line to inside the cleanup func. It is better to defer incurring the cost of running it until it is actually needed