Add Ubuntu 18.04 docker nodes, including arm64

Not replacing ubuntu1604 docker node for now. Once these have been
heavily tested and vetted, it will be much safer to move the other
docker node to Ubuntu 18.04 & v2 hardware.

Issue: RELENG-2071
Change-Id: Id3d5d06256371948f52e2454de39d87304908bb2
Signed-off-by: Eric Ball <eball@linuxfoundation.org>