Add script for new-style milestone publication

Milestones are now published directly from master and do no longer
use temporary milestone-proposed branches. Rather than using mpcut.sh
first to create the MP branch and mark bugs fixreleased, and then
mpdelivery.sh to apply the tag and upload the resulting tarball,
we'll use milestone.sh which does everything in one shot.

It calls the new ms2version.py script to translate milestone names
into version numbers suitable for tagging, rather than require that
you provide the translation as arguments.

Change-Id: If1103ebba8faee0459ed7262e3cce5bb6dc151a4