Differentiate read- and write-clusters

The ability to place a cluster into "write-only mode" is intended to
facilitate operational maneuvers, like re-provisioning a farm. The
intended workflow is:

 - Add a new cluster, in write-only mode
 - Perform a backfill, to populate the new cluster
 - Perform sanity checks, e.g. equal key counts
 - Switch the cluster to normal read+write mode

The new mode prevents farm read strategies from querying what's
effectively a known-bad set of instances while they're spinning up.
Replace all instances of '%s' with %q
Tests for ParseFarmString
TestWriteOnly
Merge pull request #20 from soundcloud/write-only

Differentiate read- and write-clusters