fix default network handling (#2029)

## Issue Addressed
#1992 and #1987, and also to be considered a continuation of #1751

## Proposed Changes
many changed files but most are renaming to align the code with the semantics of `--network` 
- remove the `--network` default value (in clap) and instead set it after checking the `network` and `testnet-dir` flags
- move `eth2_testnet_config` crate to `eth2_network_config`
- move `Eth2TestnetConfig` to `Eth2NetworkConfig`
- move `DEFAULT_HARDCODED_TESTNET` to `DEFAULT_HARDCODED_NETWORK`
- `beacon_node`s `get_eth2_testnet_config` loads the `DEFAULT_HARDCODED_NETWORK` if there is no network nor testnet provided
- `boot_node`s config loads the config same as the `beacon_node`, it was using the configuration only for preconfigured networks (That code is ~1year old so I asume it was not intended)
- removed a one year old comment stating we should try to emulate `https://github.com/eth2-clients/eth2-testnets/tree/master/nimbus/testnet1` it looks outdated (?)
- remove `lighthouse`s `load_testnet_config` in favor of `get_eth2_network_config` to centralize that logic (It had differences)
- some spelling

## Additional Info
Both the command of #1992 and the scripts of #1987 seem to work fine, same as `bn` and `vc`

lighthouse/environment/tests/environment_builder.rs

@@ -1,7 +1,7 @@
 #![cfg(test)]
 
 use environment::EnvironmentBuilder;
-use eth2_testnet_config::{Eth2TestnetConfig, DEFAULT_HARDCODED_TESTNET};
+use eth2_network_config::{Eth2NetworkConfig, DEFAULT_HARDCODED_NETWORK};
 use std::path::PathBuf;
 use types::{V012LegacyEthSpec, YamlConfig};
 
@@ -13,8 +13,8 @@ fn builder() -> EnvironmentBuilder<V012LegacyEthSpec> {
         .expect("should set logger")
 }
 
-fn eth2_testnet_config() -> Option<Eth2TestnetConfig> {
-    Eth2TestnetConfig::constant(DEFAULT_HARDCODED_TESTNET).expect("should decode mainnet params")
+fn eth2_network_config() -> Option<Eth2NetworkConfig> {
+    Eth2NetworkConfig::constant(DEFAULT_HARDCODED_NETWORK).expect("should decode mainnet params")
 }
 
 mod setup_eth2_config {
@@ -22,15 +22,15 @@ mod setup_eth2_config {
 
     #[test]
     fn update_spec_with_yaml_config() {
-        if let Some(mut eth2_testnet_config) = eth2_testnet_config() {
+        if let Some(mut eth2_network_config) = eth2_network_config() {
             let config_yaml = PathBuf::from("./tests/testnet_dir/config.yaml");
 
-            eth2_testnet_config.yaml_config = Some(
+            eth2_network_config.yaml_config = Some(
                 YamlConfig::from_file(config_yaml.as_path()).expect("should load yaml config"),
             );
 
             let environment = builder()
-                .eth2_testnet_config(eth2_testnet_config)
+                .eth2_network_config(eth2_network_config)
                 .expect("should setup eth2_config")
                 .build()
                 .expect("should build environment");