Make local provider bootstrap and destroy remove clone template files

Fixes: lp:1351004

Juju with the new flock implementation doesn't care, but forcibly destroying a local environment can leave the lock dirs around. Then if a downgrade is done, the older juju which still uses fslock thinks the lock is still held and won't start containers. This fix removes any lock files when local provider is bootstrapped and destroyed. 
As an alternative: perhaps just change the lock file name, and we could not support multiple versions of Juju running on the same system concurrently.