add rpc_public_addr_subnet config option

In case `rpc_public_addr` is not set, but autodiscovery is used, this
allows filtering the list of automatically discovered IPs to a specific
subnet.

For example, if nodes should pick *their* IP inside a specific subnet,
but you don't want to explicitly write the IP down (as it's dynamic, or
you want to share configs across nodes), you can use this option.

src/util/config.rs

@@ -85,6 +85,10 @@ pub struct Config {
 	/// Public IP address of this node
 	pub rpc_public_addr: Option<String>,
 
+	/// In case `rpc_public_addr` was not set, this can filter
+	/// the addresses announced to other peers to a specific subnet.
+	pub rpc_public_addr_subnet: Option<String>,
+
 	/// Timeout for Netapp's ping messagess
 	pub rpc_ping_timeout_msec: Option<u64>,
 	/// Timeout for Netapp RPC calls