new variant: add a config for creating ambassador variants

BUG=b:172835923
TEST=end-to-end creation of genesis

Cq-Depend: chromium:2526799
Change-Id: Id13e855a70962fd0fd6ca7e7cec903e266c9d6d7
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/dev-util/+/2526913
Reviewed-by: Paul Fagerburg <pfagerburg@chromium.org>
Commit-Queue: Matthew Ziegelbaum <ziegs@chromium.org>
Tested-by: Matthew Ziegelbaum <ziegs@chromium.org>