Bok-choy: check if mysql is running, even if it's not a daemon.

The impetus is integration with test environments, where mysql is not configured
as a service.
I prefer the "with" syntax for this kind of thing to be very clear about when the file handle is closed:

```python
with open(os.devnull, 'w') as DEVNULL:
    returncode = subprocess.call("pgrep mysqld", stdout=DEVNULL, shell=True)

return returncode == 0
```

Also note that you can pass the command in as a string instead of an array if you are using shell=True