use the default service name for shard and config server

configure node[:mongodb][:instance_name] if you want something else

I asked about why a separate ```shard``` service was configured in https://github.com/edelight/chef-mongodb/issues/174#issuecomment-30366322 and didn't here back.

With the exception of mongos, it seems like the most common use case would be to simply run a configured mongodb service. Also, on Ubuntu there is some strangeness in how services are disabled for upstart: https://tickets.opscode.com/browse/CHEF-3624 

I suppose one might want to configure multiple services on a single node for multiple mongod instances. I am not entirely how that would work with attribute configuration though. More fodder for documentation....