[System Test] Bucket flush fails with flush_wait_failed error

Build : 6.0.0-1318
Test : -test tests/analytics/test_analytics_rebalance.yml -scope tests/analytics/scope_analytics_rebalance.yml
Scale : 5
Iteration : 3

Seen this issue thrice in the above test whenever bucket flush is called.

[user:info,2018-07-16T20:21:10.889-07:00,ns_1@172.23.104.16:<0.15034.0>:ns_orchestrator:idle:560]Flushing "other-1" failed with error:
{error,{flush_wait_failed,['ns_1@172.23.104.16','ns_1@172.23.104.17',
'ns_1@172.23.104.18','ns_1@172.23.104.19'],
[]}}

Supportal link : http://supportal.couchbase.com/snapshot/dc6762750b73f8f07a9e9797b484d75f::0