Add mailing list for OpenStack Ambassadors

Currently, OpenStack Ambassadors are conducting any private communications
using a massive list of CCs. While most communications should happen
directly with the community, and on the community mailing list, this new
private mailing list will faciliate internal discussions on sensitive
topics.

Change-Id: I46122c9d688f66e910a47e7b26a73bca4cefc28b
Merge "Add mailing list for OpenStack Ambassadors"