Clean up --gossip-port argument (#7067)

--gossip-port now specifies exactly that, the gossip port to use.  The
new --gossip-host argument can be used to specify the DNS name/IP
address for gossip if --entrypoint is not supplied (when --entrypoint is
supplied, the gossip address is automatically set to the node's ip
address as observed by the entrypoint)

clap-utils/src/input_validators.rs

@@ -48,3 +48,9 @@ pub fn is_release_channel(channel: &str) -> Result<(), String> {
         _ => Err(format!("Invalid release channel {}", channel)),
     }
 }
+
+pub fn is_port(port: String) -> Result<(), String> {
+    port.parse::<u16>()
+        .map(|_| ())
+        .map_err(|e| format!("{:?}", e))
+}