pass the provider to the delete_minion_cache

It was not being passed to delete in the correct directory, because it
wasn't recreating prov_dir correctly. This fixes the issue.

Now the cache is up to date, and adds the minion to
/var/cache/salt/cloud on create, and deletes it on delete, also deletes
it if it is missing.  This could be used to key off a wheel.key.delete
in the reactor of old minions.