[autotest] Restrict wifichaos to only reimage machines to R26.

Now that suite_scheduler has support for an == branch spec, we can
enforce only R26 images will hit the chaos DUTs.

BUG=chromium-os:38962
TEST=sanity check

Change-Id: I8b8d4f5c62403e546cc7bc234df513213b218aae
Reviewed-on: https://gerrit.chromium.org/gerrit/43653
Tested-by: Alex Miller <milleral@chromium.org>
Reviewed-by: Dan Shi <dshi@chromium.org>
Commit-Queue: Alex Miller <milleral@chromium.org>
1 file changed