Enable slasher broadcast by default (#4368)

## Issue Addressed

This PR addresses issue https://github.com/sigp/lighthouse/issues/4350

## Proposed Changes

This change will enable slasher broadcast in the following cases:
No flag is passed,
`--slasher-broadcast` is passed and,
`--slasher-broadcast=true` is passed.

Only when an explicit false value is passed the slasher does not broadcast.(`--slasher-broadcast=false`).

## Additional Info

TODO

- [x] Modify CLI parsing logic
- [x] Write test

Refer to #4353 


Co-authored-by: Rahul Dogra <rahulcooldogra@gmail.com>
Co-authored-by: Gua00va <105484243+Gua00va@users.noreply.github.com>

beacon_node/src/cli.rs

@@ -792,8 +792,9 @@ pub fn cli_app<'a, 'b>() -> App<'a, 'b> {
             Arg::with_name("slasher-broadcast")
                 .long("slasher-broadcast")
                 .help("Broadcast slashings found by the slasher to the rest of the network \
-                       [disabled by default].")
-                .requires("slasher")
+                       [Enabled by default].")
+                .takes_value(true)
+                .default_value("true")
         )
         .arg(
             Arg::with_name("slasher-backend")