With the progressing implementation of the design
'design-node-security.rst', some aspects of the design
were changed or detailed. This patch brings the
design back in sync with the implementation.

Changes cover:
- Cleanup of public keys and authorized keys of
  nodes before they are readded.
- Fixing version number to 2.13, where the main
  changes in the SSH implementation are made.
- Mention new --new-ssh-key option of
  'gnt-cluster renew-crypto'.
- Mention compliance with options to suppress
  SSH file manipulation.

Signed-off-by: Helga Velroyen <helgav@google.com>
Reviewed-by: Klaus Aehlig <aehlig@google.com>