Support for configuring replicasets for authentication

There's a pull request in flight against the Puppet Labs MongoDB module to better support authentication:

https://github.com/puppetlabs/puppetlabs-mongodb/pull/138

As part of this conversation someone else has begun to add support for authenticated replicasets - which is something we need for our Ceilometer rollout. This pull requests brings in the working set of changes into our fork.