Remove Certificate Usages

- Modify text to no longer depend on Certifiocates stored in the
  overlay.
- Keep the Certificates usages in the document because self-signed
  certificates require to be stored under the CERTIFICATE_BY_USER usage,
  to check that a name is not alreday taken.
- Kept CERTIFICATE_BY_NODE even if it not used at all this -base.
- Add text saying that when using self-certificate, a node MUST store
  the certificate, as this is no longer needed for non-self-certificate
  based overlays.
- Changed peer to node when needed.
Merge pull request #168 from petithug/remove-cert-usage

Remove Certificate Usages