Update mongodb.rb

when adding a replicaset as a shard you have to use the replicaset name as the prefix

this recipe was computing a name based on rs_#{n['mongodb']['shard_name']} which results in mongos not wanting to add the shard with the following error:

mongo addshard couldn't connect to new shard socket exception

at first I thought there to be an issue with hosts/ips but after trying the commands manually and verifying ipaddresses and services running the only way to make mongo add the shards was to change the prefix to the replicaset_name