Import release-tarball script from jenkins job

This script comes from the jenkins jobs that publishes release tarballs
on https://dl.ring.cx/ring-release/tarballs/. It is best to make it
available in the source so that people can see how we generate our
tarballs and maybe use it for other purposes.

Change-Id: Ie0b302a562e92935ff97661fc85d31cdf20e0cf5
Tuleap: #254