Set tcp keepalive to 300s on Mongo machines

We started seeing socket errors and had a mongo machine die. We're not
exactly sure if the socket errors were the cause but they were the last
error in the logs. The FAQ entry below suggests lowering the tcp
keepalive.
http://docs.mongodb.org/manual/faq/diagnostics/#does-tcp-keepalive-time-affect-sharded-clusters-and-replica-sets
Merge pull request #608 from alphagov/mongo-tcp-keepalive

Set tcp keepalive to 300s on Mongo machines