testutil: Add helper functions for distributing keys

These functions will create a set of keys that map evenly across all
existing servers, thus guaranteeing that each sever will be hit at least
once when the client operates on these keys

Change-Id: Ic9acd54e210360c964e48a63b827cd56211fa569
Reviewed-on: http://review.couchbase.org/31821
Reviewed-by: Brett Lawson <brett19@gmail.com>
Tested-by: Mark Nunberg <mnunberg@haskalah.org>