contrail-api: change default "balance" option to leastconn

Current balance default "roundrobin" is not good for performance,
as with it we can end up with several "heavy" requests going
to the same worker. With "leastconn" method, the request is scheduled
to a worker with the least active sessions, therefore we avoid performance
issue when several requests are queueing at one worker.

Related-PROD: PROD-35897
Change-Id: I0098391ba92b7fd62e0326a34d8580062a708f81
1 file changed