Adding optional cluster parameters

Adds new cluster parameters with defaults as described [here](https://github.com/coreos/etcd/blob/master/Documentation/configuration.md).
Correct, it the line should be absent by default. I might be missing something, but I believe that test is covered by my updates to the default configs in fixtures by the test at https://github.com/bcicen/puppet-etcd/blob/master/spec/classes/etcd_spec.rb#L275