Only set Safe.{J,WMode} in production

As of Mongo 2.6, attempting to set Safe.J
fails if journaling is disabled in mongod.
Similarly, WMode=majority will cause problems
if a replicaset is not configured. We need
to only set these in production, where those
conditions are guaranteed to be true.
Merge pull request #487 from axw/mongo-postdial-setsafe

Only set Safe.{J,WMode} in production

As of Mongo 2.6, attempting to set Safe.J
fails if journaling is disabled in mongod.
Similarly, WMode=majority will cause problems
if a replicaset is not configured. We need
to only set these in production, where those
conditions are guaranteed to be true.