Use three agents for SI test clusters. (#5956)
Use three agents for SI test clusters. (#5956)

Summary:
Our system integration test cluster would sometimes not launch because
the AWS account ran out of resources. Currently a multi-node cluster requires
ten EC2 instances and three Elastic Load Balancers. With this change we
should only require eight instances per cluster.
meln1k/marathon,guenter/marathon,janisz/marathon,meln1k/marathon,janisz/marathon,mesosphere/marathon,gsantovena/marathon,gsantovena/marathon,gsantovena/marathon,gsantovena/marathon,mesosphere/marathon,meln1k/marathon,guenter/marathon,mesosphere/marathon,meln1k/marathon,guenter/marathon,janisz/marathon,meln1k/marathon,janisz/marathon,gsantovena/marathon,janisz/marathon,guenter/marathon,guenter/marathon,mesosphere/marathon,mesosphere/marathon