[autotest] Be able to specify LATEST build as target build.

This change allows the user to specify running a suite against
a build name in the format [builder]/LATEST. This allows for users
to just use the latest build without looking it up.

BUG=chromium:375354
TEST=Tested running from run_suite, and afe. Then created a recurring
job that worked as desired.
DEPLOY=apache
CQ-DEPEND=Iddc2ab2b0675591b53311ebe9e1692d0c0859377

Change-Id: I5c0cd9ad06bd016b539902248e193115704ae243
Reviewed-on: https://chromium-review.googlesource.com/200822
Reviewed-by: Simran Basi <sbasi@chromium.org>
Tested-by: Simran Basi <sbasi@chromium.org>
Commit-Queue: Simran Basi <sbasi@chromium.org>
3 files changed