add ssh-based uploads to releases.wikimedia.org

This changes the authentication for uploads from gpg signatures on .changes
files to being able to write to reprepro's incoming directory.

The actual transfer files from the outside happens via scp into the incoming
directory:
* authentication is provided by ssh by means of (locked down) authorized_keys
* authorization is provided on the remote side, by being in possession of an
  allowed ssh private key. Note that this also means that auditing of the key
  usage must be provided on the remote side. This is accomplished by using a
  'releases' user and protect the private key by allowing only 'sudo -u releases
  dput' to members of 'wikidev' unix group.

Also note that the public keys are locked down in what they can do on the local
side. To this end, reprepro-ssh-upload provides the necessary checks to allow
either scp into the incoming directory or to ask reprepro to process the
incoming queue.

Change-Id: I6e17b7c70e21752e558122f23436a4993feafeff
Merge "add ssh-based uploads to releases.wikimedia.org" into production