First part of ganeti sls modularization

Separate optimizations, hosts from init.sls and move users.gnt to
ganeti.user_pool. This patch also adds several other fixes and
enhancements, most noticeable ones are:
    * install ganeti from wheezy-backports
    * generate /etc/hosts in correct order (fixes gnt-node add)
    * use KVM packages from wheezy-backports (fixed versions in
      ffzg repo)