MB-8370 set number of writer to 1 when num_shard is 1
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 <a17fed27eaa842282862ff7c1b9c8395a26ac320@couchbase.com>
Tested-by: Jin Lim <c45cd80a556f432a3607171a6818cd5a074b5cd4@couchbase.com>

daverigby/ep-engine,hisundar/ep-engine,daverigby/kv_engine,teligent-ru/ep-engine,daverigby/kv_engine,daverigby/kv_engine,membase/ep-engine,sriganes/ep-engine,hisundar/ep-engine,teligent-ru/ep-engine,teligent-ru/ep-engine,hisundar/ep-engine,jimwwalker/ep-engine,daverigby/ep-engine,couchbase/ep-engine,sriganes/ep-engine,owendCB/ep-engine,hisundar/ep-engine,daverigby/kv_engine,jimwwalker/ep-engine,membase/ep-engine,jimwwalker/ep-engine,couchbase/ep-engine,couchbase/ep-engine,membase/ep-engine,sriganes/ep-engine,jimwwalker/ep-engine,owendCB/ep-engine,membase/ep-engine,daverigby/ep-engine,owendCB/ep-engine,owendCB/ep-engine,sriganes/ep-engine,daverigby/ep-engine,couchbase/ep-engine,teligent-ru/ep-engine