Support both autotest.tar and autotest.tar.bz2 at Dev server

This change favors the use of autotest.tar if it is available. The script falls
back to using autotest.tar.bz2 in other cases for backward compatibility.

BUG=chromium-os:32312
TEST=unittest & download using local devserver

Change-Id: I6c4590375a814a83a33ec84693e1cb6231752845
Reviewed-on: https://gerrit.chromium.org/gerrit/27076
Commit-Ready: Yu-Ju Hong <[email protected]>
Reviewed-by: Yu-Ju Hong <[email protected]>
Tested-by: Yu-Ju Hong <[email protected]>
3 files changed