Implement Sharding Support

This patch aims to support MongoDB sharding feature.

* Enable Sharding in MongoDB configuration file if enabled (disabled by
  default).
* Add a Type and a Provider to manage a MongoDB Shard resource.
* Add tests/sharding.pp to show an example of usage.
* Update the documentation about MongoDB Sharding usage.
* Unit tests for the new Type and the new Provider.

Conflicts:
	manifests/server.pp