Ensure rs.conf() is run on the proper mongod process

Currently, rs.conf() can only be run on 127.0.0.1:27017.
When running a node in shardsvr mode, the mongod process
listens on port 27018, hence the rs.conf() check fails.
Add support for mongos configuration
Implement Sharding Support

This patch aims to support MongoDB sharding feature.

* Add a Type and a Provider to manage a MongoDB Shard resource.
* Add tests/sharding.pp to show an example of usage.
* Unit tests for the new Type and the new Provider.