added ns_cluster_membership:active_nodes/1
added ns_cluster_membership:active_nodes/1

Change-Id: I1d18b6c955e095e4f46c63ad34acef22ff3c23bd
Reviewed-on: http://review.couchbase.org/7719
Tested-by: Aliaksey Kandratsenka <340b8e09ca65cd3fc686427fcfed17e87eaf61e2@gmail.com>
Reviewed-by: Bin Cui <ed18693fff32c00e22495f4877a3b901bed09041@gmail.com>

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