jgroups gossip discovery example configuration files

Hello, I've added gossip discovery example configuration files similarly to the EC2 configuration files in https://github.com/wildfly/wildfly/pull/4886

The reason to have this alternative method is that it would allow for running in any environment, even without internet access. As well it's very convenient for testing purposes.

FYI I'm not yet preparing mod_cluster configuration files because it seems it's not prepared to work with undertow yet.