Add 'boxbackup' role

- boxbackup (http://boxbackup.org/) is the primary backup solution for
  ginas. Box Backup creates remote, encrypted, versioned backups of files
  with minimal effort;

- by default boxbackup is not enabled. You can enable it for all or
  selected group of hosts by specifying a fully qualified domain name of
  boxbackup server (in 'boxbackup' variable);

- if the boxbackup server is within the ginas cluster, it will be
  automatically configured as such. Certificate Authority will be created
  and managed on Ansible Controller host ('secret' directory is required
  for that, see the 'secret' role for more information);

- hosts which are not boxbackup servers will be automatically configured
  as clients and an account for each will be created on specified
  boxbackup server (if it is managed by ginas). Certificates will be
  automatically signed and distributed. Each client encryption key will be
  automatically archived in a separate directory on Ansible Controller (in
  'secret' directory);

- boxbackup role proviedes backup and restore tasks which are executed
  by 'backup.sh' and 'restore.sh' scripts to safely recreate configuration
  after reinstall. Files backed up by boxbackup itself are not recovered
  automatically;