Fix inconsistency in sandesh c++/python client code in

collector service subscription with discovery

c++ sandesh client:
- subscribes for collector service irrespective of whether the collector
list is provided in the InitGenerator(), if CollectorSubFn is not NULL.

python sandesh client:
- subscribes for collector service only if the collector list is empty
in init_generator()

Modified the sandesh c++ library to subscribe for the collector service
only if the collector list is empty.

Change-Id: I57a2c8f2612df0b8b8f4ebef72d2dd7f62d8f626
Closes-Bug: #1531524

library/cpp/sandesh_client.cc

@@ -102,7 +102,9 @@ void SandeshClient::Initiate() {
     sm_->SetAdminState(false);
     if (primary_ != Endpoint())
         sm_->SetCandidates(primary_,secondary_);
-    if (csf_!=0){
+    // subscribe for the collector service only if the collector list
+    // is not provided by the generator.
+    else if (csf_ != 0) {
         SANDESH_LOG(INFO, "Subscribe to Discovery Service for Collector" );
         csf_(g_vns_constants.COLLECTOR_DISCOVERY_SERVICE_NAME, 2,
             boost::bind(&SandeshClient::CollectorHandler, this, _1));