account for behaviour prior to https://github.com/rcbops-cookbooks/mysql-openstack/pull/47

fixes https://github.com/rcbops/chef-cookbooks/issues/515
addresses Issue #DE249

Prior to https://github.com/rcbops-cookbooks/mysql-openstack/pull/47, binlogging was configured in the main my.cnf using the ['mysql']['tunable']['log_bin'] as a toggle.  

To account for any installs that were doing it this way, we need to unset this attribute to ensure that config gets removed from the main my.cnf, and then ensure we drop in the conf.d file prior to including the mysql::server recipe (which restarts mysql due to the changed my.cnf).  This ensures that we are not disabling binlogging at any point.