Update ubuntu version from 18.10 to 18.04 (#3237)
Update ubuntu version from 18.10 to 18.04 (#3237)

Changes Ubuntu version from 18.10 to more stable 18.04 with latest amd64 digest tag

mesosphere/dcos-commons,mesosphere/dcos-commons,mesosphere/dcos-commons,mesosphere/dcos-commons,mesosphere/dcos-commons