Use RPC to get the port instead of parsing the argument

[sgkim126]

Currently, the client thinks the port of the node is 3485 if there is no argument.

codechain-dashboard/client/src/process/mod.rs

Line 622 in 2f061c2

fn parse_flags(args: &[String]) -> (u16, String) {

However, the port can be set through config.toml.
Please make it use net_port RPC.