client: Introduce --sentry-nodes flag (#4285)

* client/authority-discovery: Add smoke tests for intervall_at

* client/authority-discovery: Fix interval_at to fire on start

* .maintain/sentry-node: Update docker compose file

* client: Introduce --sentry-nodes flag

Enable operators to specify the public addresses of sentry nodes infront
of a validator node so that the validator node can announce the sentry
node addresses instead of its own public addresses on the DHT via the
authority discovery module.

* client/authority-discovery: Break lines at 100 characters

Limit line length to 100 instead of 120 characters.

substrate/client/cli/src/lib.rs

@@ -674,12 +674,14 @@ fn fill_network_configuration(
 	config.boot_nodes.extend(cli.bootnodes.into_iter());
 	config.config_path = Some(config_path.to_string_lossy().into());
 	config.net_config_path = config.config_path.clone();
-	config.reserved_nodes.extend(cli.reserved_nodes.into_iter());
 
+	config.reserved_nodes.extend(cli.reserved_nodes.into_iter());
 	if cli.reserved_only {
 		config.non_reserved_mode = NonReservedPeerMode::Deny;
 	}
 
+	config.sentry_nodes.extend(cli.sentry_nodes.into_iter());
+
 	for addr in cli.listen_addr.iter() {
 		let addr = addr.parse().ok().ok_or(error::Error::InvalidListenMultiaddress)?;
 		config.listen_addresses.push(addr);

substrate/client/cli/src/params.rs

@@ -177,6 +177,14 @@ pub struct NetworkConfigurationParams {
 	#[structopt(long = "reserved-only")]
 	pub reserved_only: bool,
 
+	/// Specify a list of sentry node public addresses.
+	#[structopt(
+		long = "sentry-nodes",
+		value_name = "URL",
+		conflicts_with_all = &[ "sentry" ]
+	)]
+	pub sentry_nodes: Vec<String>,
+
 	/// Listen on this multiaddress.
 	#[structopt(long = "listen-addr", value_name = "LISTEN_ADDR")]
 	pub listen_addr: Vec<String>,