cbuildbot.py: add metadata_dump command line option

Cherry-pick of https://chromium-review.googlesource.com/#/c/205221/

In order to land upcoming metadata_dump changes, it is necessary that
cbuildbot.py be able to accept a metadata_dump argument without
complaint, even if it does not use this argument.

BUG=chromium:394978
TEST=None

Change-Id: Ic4eaa943072f3de95a3ccda702e9c9ff8e0ad316
1 file changed