handling of my.cnf config file is now optional

This adds a parameter (default value is like old behavior) so that the
my.cnf file isn't managed (created/updated) by the mysql module at all
which is necessary for our environment. We need to set all parameters by
ourself. If we don't set a parameter then the default from the mysqld
binary should be used instead of any default parameter the mysql module
provides us.
Merge pull request #132 from Savar/manage_config_file_parameter

handling of my.cnf config file is now optional