Occasional erros in the mysql driver

Hello,

We are getting the following error in gnocchi:

2018-12-13 10:01:05,723 [3173] ERROR cotyledon._utils: Unhandled exception
Traceback (most recent call last):
  File "/usr/lib/python2.7/site-packages/cotyledon/_utils.py", line 84, in exit_on_exception
    yield
  File "/usr/lib/python2.7/site-packages/cotyledon/_service.py", line 139, in _run
    self.run()
  File "/usr/lib/python2.7/site-packages/gnocchi/cli.py", line 137, in run
    self._run_job()
  File "/usr/lib/python2.7/site-packages/gnocchi/cli.py", line 261, in _run_job
    lock.release()
  File "/usr/lib/python2.7/site-packages/tooz/drivers/mysql.py", line 102, in release
    cause=e)
  File "/usr/lib/python2.7/site-packages/tooz/utils.py", line 225, in raise_with_cause
    excutils.raise_with_cause(exc_cls, message, *args, **kwargs)
  File "/usr/lib/python2.7/site-packages/oslo_utils/excutils.py", line 143, in raise_with_cause
    six.raise_from(exc_cls(message, *args, **kwargs), kwargs.get('cause'))
  File "/usr/lib/python2.7/site-packages/six.py", line 718, in raise_from
    raise value
ToozError: (0, '')

I looked around the source code and I suspect that this is caused by a leaked mysql cursor. Tooz creates cursors using `with self._conn as cur:`.
However, if you look at the source code of PyMySQL you will see that the connection doesn't close the cursor:

https://github.com/PyMySQL/PyMySQL/blob/master/pymysql/connections.py#L499-L508

The proper way of using cursors is exemplified in the PyMySQL examples:

with connection.cursor() as cursor:
    # do stuff

The fix would be to replace `with self._conn as cur:` with
`with self._conn.cursor() as cur:` in the mysql driver.