[bp] HTTP status 500 from REST call post to engageCluster2, on /controller/addNode

Intermittently, upgrade test fails due to 500 returned from /engateCluster2 on /controller/addNode.

Here is steps to reproduce.(But not happen always)
1. Have a 4.5 cluster with (2 kv, 2 index, 2 query)
2. Add a Vulcan kv node by /controller/addNode on 4.5 cluster

Expected : no error
Actual : Failed due to 500 from Vulcan node being added.

Request : POST http://172.17.0.4:8091/controller/addNode
Resposne: ["Prepare join failed. Got HTTP status 500 from REST call post to http://172.17.0.13:8091/engageCluster2. Body was: \"\\\"Unexpected server error, request logged.\\\"\""]

Please note that 172.17.0.4 is the entry point of REST call (4.5 GA) and 172.17.0.13 is the Vulcan node being added.