cbuildbot: Improve argument handling schedule scheduling paygen tryjobs.
cbuildbot: Improve argument handling schedule scheduling paygen tryjobs.

When scheduling payloads tryjobs, there are a couple of odd behaviors.

1) We don't normally add a CL # as part of the job, so there is no need
   to warn that no CL # was specified.

2) If --channel isn't used, the job will run, but do nothing useful. We
   should expect a --channel argument when scheduling a payloads build.

So.... now we do these things.

BUG=chromium:357734
TEST=Ran existing tests + schduled paygen runs manually.

Change-Id: Ia49c86ce06e5d8956716f6c13d64732f2ad3f004
Reviewed-on: https://chromium-review.googlesource.com/200304
Tested-by: Don Garrett <546ec366615570e68497e3c5df9092106a9808f4@chromium.org>
Reviewed-by: Yu-Ju Hong <52dc720c411f8e2156e608c1a8b894a917489667@chromium.org>
Commit-Queue: Don Garrett <546ec366615570e68497e3c5df9092106a9808f4@chromium.org>

bpsinc-native/src_third_party_chromite,bpsinc-native/src_third_party_chromite,chadversary/chromiumos.chromite,chadversary/chromiumos.chromite,bpsinc-native/src_third_party_chromite