cbuildbot: Add api to --buildbucket-id.

Old enough branches will fail because cbuildbot_launch will pass along
the unknown option --buildbucket-id. This adds an API version to the
option to prevent this.

I'm using for a version (REEXEC_API_GOMA) added after buildbucket-id,
but that means that some branches which could understand
--buildbucket-id won't get it.

BUG=chromium:770213
TEST=Manual cbuildbot_launch invocation with factory-rambi-6420.B

Change-Id: I0bbdb0f87c5260b1d66f1f56b8cc2084511b5d21
Reviewed-on: https://chromium-review.googlesource.com/821611
Commit-Ready: Don Garrett <dgarrett@chromium.org>
Tested-by: Don Garrett <dgarrett@chromium.org>
Reviewed-by: Aviv Keshet <akeshet@chromium.org>
diff --git a/scripts/cbuildbot.py b/scripts/cbuildbot.py
index 4af5828..d787201 100644
--- a/scripts/cbuildbot.py
+++ b/scripts/cbuildbot.py
@@ -506,6 +506,7 @@
   group.add_option('--remote-trybot', action='store_true', default=False,
                    help='Indicates this is running on a remote trybot machine')
   group.add_option('--buildbucket-id',
+                   api=constants.REEXEC_API_GOMA, # Approximate.
                    help='The unique ID in buildbucket of current build '
                         'generated by buildbucket.')
   group.add_remote_option('--remote-patches', action='split_extend', default=[],