MB-8370 set number of writer to 1 when num_shard is 1

Without this fix, the number of writer would be set to 0 if
num_shard is 1

Change-Id: I5022ca77ba476019daa6eec3d4bd60dd1bf36532
Reviewed-on: http://review.couchbase.org/26650
Reviewed-by: Michael Wiederhold <mike@couchbase.com>
Tested-by: Jin Lim <jin@couchbase.com>