Use MySQLdbs read_default_file

Removed custom config parsing and some unused code. Let's you specify a config_file that should be read instead of ~/.my.cnf

These changes are quite extensive, it will break config-parsing where non-mysql-compliant formats where used.

PS: this is a re-apply of this PR: https://github.com/ansible/ansible/pull/6295 I Hope nothing did brake while cherry picking the commits.