[system test]: swap rebalance hang in online upgrade from 1.8.1 to 2.0.1-168

Rebalance hang when doing swap rebalance by add node 2.0.1-168 to cluster and remove one 1.8.1 out of
cluster.

Environment:

1.8.1 nodes

1 = ec2-50-16-9-117.compute-1.amazonaws.com

1. short public IP 50.16.9.117
2. 10.149.28.28
   2 = ec2-54-242-127-186.compute-1.amazonaws.com
3. short public IP 54.242.127.186
4. 10.149.19.204
   3 = ec2-23-20-192-140.compute-1.amazonaws.com <== this node is remove to do 1st swap rebalance
5. short public IP 23.20.192.140
6. 10.149.27.66
   4 = ec2-204-236-252-180.compute-1.amazonaws.com
7. short public IP 204.236.252.180
8. 10.149.21.76

Install couchbase server 2.0.1-168 in below node
5 = ec2-50-16-32-181.compute-1.amazonaws.com <== 2.0.1-167 node added to do 1st swap rebalance for online upgrade

1. short public IP 50.16.32.181
2. 10.149.8.5

Doing swap rebalance. Just in few minutes when I check vbuckets in node 66, I saw it is 0.

Diags will be posted soon.