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. 
I don't think this is valid. There may be multiple users on a system using the local provider, and they all use the same lock file. We shouldn't just be removing it like that.

The old code is broken *anyway*, and there's nothing we can do about that. I think what we can do is:
  - in container.lock.Lock: os.MkdirAll, os.Open, flockLock
  - in container.lock.Unlock: os.Remove, lock.lockFile.Close

The remove will unlink the directory, which is safe because we hold the lock. The Close will release the lock (or you can explicitly flockUnlock as you do now).