Make 'label' names more meaningful in nightly tests.

The nightly crosperf tests currently use the label 'test_image' for
the images that we build or that the trybot built.  This is not
a very informative name.  This CL tries to make the names a least a
little bit more useful.

BUG=None
TEST=Tested by hand in role account directory tree.

Change-Id: Ib88f711ec96ec5cb71efebae79a76e6e8ebc784a
Reviewed-on: https://chrome-internal-review.googlesource.com/237796
Commit-Ready: Caroline Tice <cmtice@google.com>
Tested-by: Caroline Tice <cmtice@google.com>
Reviewed-by: Luis Lozano <llozano@chromium.org>
2 files changed