update ubuntu image to 16.04.201808060 (#3637)
update ubuntu image to 16.04.201808060 (#3637)


Azure/acs-engine,Azure/acs-engine,jackfrancis/acs-engine,jackfrancis/acs-engine,Azure/acs-engine,Azure/acs-engine,jackfrancis/acs-engine,jackfrancis/acs-engine,jackfrancis/acs-engine,Azure/acs-engine