Tidy Eth2Config generation at runtime (#912)

* Move the codes that loads Eth2Config from config to environment

* Move the codes that setups Eth2Config for testnet

* Move the codes that creates a new ChainSpec

* Remove unused `mut`

* Reduce local variable number

* Remove unused outputs of config::get_configs()

* Change the method name from plural to singular

* DRY the const `ETH2_CONFIG_FILENAME`

* Add comments

* Remove unnecessary blank line

* cargo fmt

* Add tests for EnvironmentBuilder::setup_eth2_config()

* Remove the comment that have been fixed

* Reduce local variable

* Remove redundant local variable

* Remove prysm-specific codes

Now the spec is in the eth2-testnets repo

beacon_node/src/cli.rs

@@ -308,14 +308,5 @@ pub fn cli_app<'a, 'b>() -> App<'a, 'b> {
                     .required(true)
                     .help("A file from which to read the state"))
             )
-            /*
-             * `prysm`
-             *
-             * Connect to the Prysmatic Labs testnet.
-             */
-            .subcommand(SubCommand::with_name("prysm")
-                .about("Connect to the Prysmatic Labs testnet on Goerli. Not guaranteed to be \
-                    up-to-date or functioning.")
-            )
         )
 }