cbuildbot: split out dedicated paygen builder

Move the paygen specific builder into the existing release builder module.

Can't drop PAYLOADS_TYPE as it's used to do some argument/sanity checking
in other parts of the code.

BUG=brillo:134, chromium:299943
TEST=`./cbuildbot/run_tests` passes

Change-Id: Idb0d7be9dc3f9f248eebc8af5a830d11e00f6272
Reviewed-on: https://chromium-review.googlesource.com/262662
Reviewed-by: Don Garrett <dgarrett@chromium.org>
Commit-Queue: Mike Frysinger <vapier@chromium.org>
Tested-by: Mike Frysinger <vapier@chromium.org>
4 files changed