Get replicaSet using os.getenv()

- Previously we assumed all environments would have a replicaset called 'production'
- This broke ci-new when it attemped to attach to a different set
- Adding in the variable allows us to override the value of the replicaset in various jenkins jobs
Merge pull request #229 from alphagov/replica_set_as_variable

Get replicaSet using os.getenv()