Packet filter cli option (#2523)

## Issue Addressed

N/A

## Proposed Changes

Adds a cli option to disable packet filter in `lighthouse bootnode`. This is useful in running local testnets as the bootnode bans requests from the same ip(localhost) if the packet filter is enabled.

beacon_node/src/cli.rs

@@ -45,6 +45,12 @@ pub fn cli_app<'a, 'b>() -> App<'a, 'b> {
                        --subscribe-all-subnets to ensure all attestations are received for import.")
                 .takes_value(false),
         )
+        .arg(
+            Arg::with_name("disable-packet-filter")
+                .long("disable-packet-filter")
+                .help("Disables the discovery packet filter. Useful for testing in smaller networks")
+                .takes_value(false),
+        )
         .arg(
             Arg::with_name("zero-ports")
                 .long("zero-ports")