If bots are specified manually on command line OR in PRESUBMIT.py,
then old behavior remains, namely triggering just those bots.

Otherwise, git cl try will now trigger CQ Dry Run. This is a good
default and also resolves several bugs:

 * https://crbug.com/581150 where git cl try used to load cq.cfg
   from infra/config, but that's not always correct for all projects.

 * https://crbug.com/625697 where even if cq.cfg is in default location.
   it might be outdated.

 * https://crbug.com/585237 where git cl try would ignore special
   in CL description CQ_INCLUDE_TRYBOTS, which is processed by CQ,
   but not git cl try.

BUG=625697,585237,581150
R=sergiyb@chromium.org,emso@chromium.org

Review-Url: https://codereview.chromium.org/2147563003