Add privacy option (#2016)

Adds a `--privacy` CLI flag to the beacon node that users may opt into. 

This does two things:
- Removes client identifying information from the identify libp2p protocol
- Changes the default graffiti to "" if no graffiti is set.

beacon_node/src/config.rs

@@ -305,6 +305,8 @@ pub fn get_config<E: EthSpec>(
         }
 
         graffiti.as_bytes()
+    } else if cli_args.is_present("private") {
+        b""
     } else {
         lighthouse_version::VERSION.as_bytes()
     };
@@ -577,6 +579,10 @@ pub fn set_network_config(
         config.upnp_enabled = false;
     }
 
+    if cli_args.is_present("private") {
+        config.private = true;
+    }
+
     Ok(())
 }