[r=axwalk],[bug=1247152] cmd/juju: delete .jenv if bootstrap fails
If bootstrap fails, and the .jenv file did not previously exist, then Juju will attempt to destroy the environment in case of partial creation. If this succeeds, the .jenv file is destroyed. If it fails, an error message is logged; we do not currently attempt to forcefully remove the environment.
NOTE: if an environment is already prepared, say by running sync-tools, then a failed bootstrap will not attempt to destroy the environment.