check validity of key and tls in a server-list

check validity of key and tls in a server-list

If a `key` or `tls` is associated to an IP address inside a server-list,
only the `tls` existence in the configuration was checked. Also, if
`key` or `tls` is associated to a named server-list inside a
server-list, there was no check at all.

Add the check for making sure a `key` is defined in the configuration,
as well as the check for `key` and `tls` when used on a named
server-list.