Change default value for `node['mysql-multi']['slaves']` to `%w()`

Update summary:
- a91ae5d Changes the default value for `node['mysql-multi']['slaves']` to `%w()`.  This allows `mysql-multi::mysql_master` to converge and run if no slaves are set. This could be used in a chef-solo situation to setup the master before configuring the slaves (Heat, CloudFormation)
- b2abdb6 Bumps the version of CentOS to test with from 6.4 to 6.5