I find CDH does not support Ubuntu 18.04 yet, and plan to support ? I hope Ubuntu 18.04 can be supported, and Canonical extends Ubuntu 18.04 support to 10 years, really LTS. So I hope CDH could support Ubuntu 18.04.
Ubuntu 18.04 is now supported with CDH 6.2 release, see Release Notes
As per, lastest cdh 6.1 support 16.04 LTS (Xenial). For reference, Cloudera Enterprise 6.1.x Supported Operating Systems
Hope that helps.
About the future roadmap, I obviously don't have the visibility. It will be better known to product teams.
Will CDH 5 also going to support Ubuntu 18.04?
There are no plans to add support to C5 releases. See currently supported OS versions