The jenkins user has no home directory, so "su" does not directly work

repeat these steps for every schroot

• Create a ssh key
  • sudo -u jenkins ssh-keygen
• Add it to gitlab/gitorious/etc
  • sudo -u jenkins cat /var/lib/jenkins/.ssh/id_rsa.pub
• the buildserver has to be in the known hosts list, so we connect to it to be questioned about it
  • sudo -u jenkins ssh git.hb.dfki.de
• add the key to the ssh-agent
  • sudo -u jenkins sh -c "ssh-agent ssh-add /var/lib/jenkins/.ssh/id_rsa"