Searches for existing server instances with matching tags.  To match,
        the existing instances must also be "running".

def find_existing(self):
        """
        Searches for existing server instances with matching tags.  To match,
        the existing instances must also be "running".

        """
        instances = self.consul.find_servers(self.tags)
        maxnames = len(instances)
        while instances:
            i = instances.pop(0)
            server_id = i[A.server.ID]
            if self.namespace.add_if_unique(server_id):
                log.info('Found existing server, %s' % server_id)
                self.server_attrs = i
                break
            if len(self.namespace.names) >= maxnames:
                break
            instances.append(i)