marge-bot: make add-part-of configurable

WARNING: I HAVE NOT TESTED THIS!

linux-media does not want the "Part-of" tag, it would make Linus unhappy.

We don't want to change the value of others. Store the value in a configurable environment variable and let run_marge.sh pick it up.

By default the value is True, so there is no changes for other projects

Signed-off-by: Ricardo Ribalda ribalda@chromium.org