gnocchi cache lock is useless when cache is global

when checking cache, we[1]:

1. check cache
2. if missing, create lock,
3. check cache again to see it's not created by another publisher
4. set cache
5. discard lock

it's debatable how useful this is since the lock is local. if the cache is global (redis), the lock only protects the another thread from attempting the same.

[1] https://github.com/openstack/ceilometer/blob/0e5c2cfef1e2fb1eca3c688cbb1bf9ae86db308c/ceilometer/publisher/gnocchi.py#L418