Adding arbiter addition to replicaset

This change lets you add an arbiter to the replicaset in this way:

```
mongodb_replset { 'myrepl':
  ensure => present,
  members => ['host1', 'host2'],
  arbiter => 'arbiter_host:30000',
}
```

I'm a newbie dealing with providers and types, so probably the code is not appropriate. I had some problems like i didn't find how or when the setters of new properties are called, so to solve the issue i just called it from line 61.

I guess this is not correct, so if you can guide me about how to solve this issue, it would be great.