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stating we should try to emulate `https://github.com/eth2-clients/eth2-testnets/tree/master/nimbus/testnet1` it looks outdated (?)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

common/directory/src/lib.rs

@@ -1,5 +1,5 @@
 use clap::ArgMatches;
-pub use eth2_testnet_config::DEFAULT_HARDCODED_TESTNET;
+pub use eth2_network_config::DEFAULT_HARDCODED_NETWORK;
 use std::fs::{self, create_dir_all};
 use std::path::{Path, PathBuf};
 
@@ -19,13 +19,13 @@ pub const CUSTOM_TESTNET_DIR: &str = "custom";
 /// Tries to get the name first from the "network" flag,
 /// if not present, then checks the "testnet-dir" flag and returns a custom name
 /// If neither flags are present, returns the default hardcoded network name.
-pub fn get_testnet_name(matches: &ArgMatches) -> String {
-    if let Some(testnet_name) = matches.value_of("network") {
-        testnet_name.to_string()
+pub fn get_network_dir(matches: &ArgMatches) -> String {
+    if let Some(network_name) = matches.value_of("network") {
+        network_name.to_string()
     } else if matches.value_of("testnet-dir").is_some() {
         CUSTOM_TESTNET_DIR.to_string()
     } else {
-        eth2_testnet_config::DEFAULT_HARDCODED_TESTNET.to_string()
+        eth2_network_config::DEFAULT_HARDCODED_NETWORK.to_string()
     }
 }
 
@@ -54,7 +54,7 @@ pub fn parse_path_or_default_with_flag(
         arg,
         PathBuf::new()
             .join(DEFAULT_ROOT_DIR)
-            .join(get_testnet_name(matches))
+            .join(get_network_dir(matches))
             .join(flag),
     )
 }