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stating we should try to emulate `https://github.com/eth2-clients/eth2-testnets/tree/master/nimbus/testnet1` it looks outdated (?)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`

validator_client/src/config.rs

@@ -2,7 +2,7 @@ use crate::{http_api, http_metrics};
 use clap::ArgMatches;
 use clap_utils::{parse_optional, parse_required};
 use directory::{
-    get_testnet_name, DEFAULT_HARDCODED_TESTNET, DEFAULT_ROOT_DIR, DEFAULT_SECRET_DIR,
+    get_network_dir, DEFAULT_HARDCODED_NETWORK, DEFAULT_ROOT_DIR, DEFAULT_SECRET_DIR,
     DEFAULT_VALIDATOR_DIR,
 };
 use eth2::types::Graffiti;
@@ -44,12 +44,12 @@ pub struct Config {
 impl Default for Config {
     /// Build a new configuration from defaults.
     fn default() -> Self {
-        // WARNING: these directory defaults should be always overrided with parameters
-        // from cli for specific networks.
+        // WARNING: these directory defaults should be always overwritten with parameters from cli
+        // for specific networks.
         let base_dir = dirs::home_dir()
             .unwrap_or_else(|| PathBuf::from("."))
             .join(DEFAULT_ROOT_DIR)
-            .join(DEFAULT_HARDCODED_TESTNET);
+            .join(DEFAULT_HARDCODED_NETWORK);
         let validator_dir = base_dir.join(DEFAULT_VALIDATOR_DIR);
         let secrets_dir = base_dir.join(DEFAULT_SECRET_DIR);
         Self {
@@ -91,13 +91,13 @@ impl Config {
 
         config.validator_dir = validator_dir.unwrap_or_else(|| {
             default_root_dir
-                .join(get_testnet_name(cli_args))
+                .join(get_network_dir(cli_args))
                 .join(DEFAULT_VALIDATOR_DIR)
         });
 
         config.secrets_dir = secrets_dir.unwrap_or_else(|| {
             default_root_dir
-                .join(get_testnet_name(cli_args))
+                .join(get_network_dir(cli_args))
                 .join(DEFAULT_SECRET_DIR)
         });