Update mongodb.rb

[dave-leblanc]

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

[josephholsten]

I totally support us being more flexible with the shard name. This is backwards incompatible, so I'm adding it to #308.

This code does what it says on the box, but you don't need the string interpolation. Instead of:

key = "#{n['mongodb']['replicaset_name']}"

We may simply:

key = n['mongodb']['replicaset_name']

[josephholsten]

Oh, looks like this is a dupe of #6

@jamesonjlee this seems safe to close

[jamesonjlee]

noting as to close with #215