Add Ubuntu18 jobs.

Ubuntu18 will now run all jobs currently on Ubuntu17 except Valgrind.

Change-Id: Icfdf798d43917beed0bfa1c96763d5b8ff6eb7c9
Reviewed-on: https://skia-review.googlesource.com/c/182200
Commit-Queue: Kevin Lubick <kjlubick@google.com>
Auto-Submit: Ben Wagner <benjaminwagner@google.com>
Reviewed-by: Kevin Lubick <kjlubick@google.com>