Allow MySQL check configuration via a mysql config file

[micktwomey]

For more complex configuration (e.g. specifying SSL certificates) it's useful to use a mysql.cnf instead of specifying flags by hand.

For example I have a config like this:

[client]
default-character-set=utf8
database=dbname
user=username
password="password"
host=hostname.example.com
ssl-ca=/path/to/mysql-ssl-ca-cert.pem

I can then run the check using:

init_config:

instances:
   - defaults_file: /path/to/my.cnf

This (for example) allows datadog to collect metrics on externally hosted mysql instances over SSL.

[remh]

You can get rid of this line. This method is for backward compatibility to use old style configuration that was using datadog.conf to configure checks which is now deprecated.
default_files was not supported at that time so there is no need to add it now.

[remh]

@micktwomey
It looks good!
Can you fix the line I pointed out and also rebase your branch so it's merged with master as we updated this check yesterday ?
Thanks!