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.
Is the exit code for pgrep always 0?   If not, I think this needs to catch the error raised by the suprocess call (https://docs.python.org/2/library/subprocess.html#subprocess.CalledProcessError)

My understanding is that pgrep will return 0 if it finds a matching process, and 1 if it does not.  Since subprocess.check_call raises an error with return code 1, won't the whole test process just error out rather than returning the message that mysql isn't running?