Added 'monkeysphere' role

- monkeysphere (http://web.monkeysphere.info/) allows you to use
  PGP/GnuPG public keys to authorize access to SSH and HTTPS. Currently
  only SSH access is supported in 'monkeysphere' role;

- by default monkeysphere is disabled on all hosts in the cluster, look
  in defaults/main.yml for information how to enable and use it;

- currently there is no support for adding user UIDs automatically,
  it's planned for 'users' role. By default user UIDs are looked up in
  /etc/monkeysphere/authorized_user_ids/ directory;

- this role works really well with 'sks' role if you want to set up
  small cluster of internal keyservers for authentication;