Associates the provided keywords with the client.

def AddClientKeywords(self, client_id, keywords):
    """Associates the provided keywords with the client."""
    if client_id not in self.metadatas:
      raise db.UnknownClientError(client_id)

    for kw in keywords:
      self.keywords.setdefault(kw, {})
      self.keywords[kw][client_id] = rdfvalue.RDFDatetime.Now()