This commit makes it an error to start a node where either of the
server contexts (xpack.security.transport.ssl and
xpack.security.http.ssl) meet either of these conditions:

1. The server lacks a certificate/key pair (i.e. neither
   ssl.keystore.path not ssl.certificate are configured)
2. The server has some ssl configuration, but ssl.enabled is not
   specified. This new validation does not care whether ssl.enabled is
   true or false (though other validation might), it simply makes it
   an error to configure server SSL without being explicit about
   whether to enable that configuration.

We added a few tests with invalid config while this PR was stagnant