provide default value for db_shards bucket parameter. MB-3420
provide default value for db_shards bucket parameter. MB-3420

Upgrades from old configs where this parameter is missing will not
work otherwise.

Change-Id: I9b0d896ba1ed9bd6990a6f95c25feb6140022632

t3rm1n4l/ns_server,mschoch/ns_server,vzasade/ns_server,ceejatec/ns_server,daverigby/ns_server,pavel-blagodov/ns_server,vzasade/ns_server,nimishzynga/ns_server,pavel-blagodov/ns_server,membase/ns_server,nimishzynga/ns_server,mschoch/ns_server,mschoch/ns_server,mschoch/ns_server,membase/ns_server,t3rm1n4l/ns_server,membase/ns_server,vzasade/ns_server,membase/ns_server,nimishzynga/ns_server,daverigby/ns_server,membase/ns_server,mschoch/ns_server,ceejatec/ns_server,t3rm1n4l/ns_server,daverigby/ns_server,mschoch/ns_server,ceejatec/ns_server,vzasade/ns_server,daverigby/ns_server,ceejatec/ns_server,t3rm1n4l/ns_server,ceejatec/ns_server,pavel-blagodov/ns_server,ceejatec/ns_server,nimishzynga/ns_server,daverigby/ns_server,pavel-blagodov/ns_server,nimishzynga/ns_server,t3rm1n4l/ns_server,membase/ns_server,nimishzynga/ns_server