Add 'openvz' role

- this role adds OpenVZ support in Debian Wheezy using a custom kernel
  from openvz.org. There might be some changes needed in GRUB
  configuration to enable new kernel, you should check on which menu
  entry new kernel will be installed;

- openvz role will not reboot your server automatically, instead it will
  send an email message using local SMTP server to root account, which
  should be forwarded to your user account by Postfix role;

- there's currently no full support for OpenVZ containers in ginas
  itself, at the moment ferm gives me some trouble, probably with
  conntrack. Updates will be commited at a later date;