Fixed params.pp

There are two issues on Debian based distributions using
mongodb.org packages:

```puppet
class {'::mongodb::globals':
manage_package_repo => true,
}->
class {'::mongodb::server':}->
class {'::mongodb::client':}
```

### $config is undef for mongodb config unter /etc

Error: Parameter path failed on File[undef]: File paths must be fully qualified, not 'undef' at /etc/puppet/modules/mongodb/manifests/server/config.pp:83

### $service_name is `mongodb` instead of `mongod`

Error: /Stage[main]/Mongodb::Server::Service/Service[mongodb]: Could not evaluate: Could not find init script or upstart conf file for 'mongodb'
Error: /Stage[main]/Mongodb::Server::Service/Service[mongodb]: Failed to call refresh: Could not find init script or upstart conf file for 'mongodb'
Error: /Stage[main]/Mongodb::Server::Service/Service[mongodb]: Could not find init script or upstart conf file for 'mongodb'
Warning: /Stage[main]/Mongodb::Server::Service/Mongodb_conn_validator[mongodb]: Skipping because of failed dependencies
Warning: /Stage[main]/Mongodb::Server/Anchor[mongodb::server::end]: Skipping because of failed dependencies
Merge pull request #144 from synyx/master

Fixed params.pp