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stating we should try to emulate `https://github.com/eth2-clients/eth2-testnets/tree/master/nimbus/testnet1` it looks outdated (?)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

@@ -1,7 +1,7 @@
 //! A helper library for parsing values from `clap::ArgMatches`.
 
 use clap::ArgMatches;
-use eth2_testnet_config::Eth2TestnetConfig;
+use eth2_network_config::Eth2NetworkConfig;
 use ssz::Decode;
 use std::path::PathBuf;
 use std::str::FromStr;
@@ -16,9 +16,9 @@ pub const BAD_TESTNET_DIR_MESSAGE: &str = "The hard-coded testnet directory was
 pub fn parse_testnet_dir(
     matches: &ArgMatches,
     name: &'static str,
-) -> Result<Option<Eth2TestnetConfig>, String> {
+) -> Result<Option<Eth2NetworkConfig>, String> {
     let path = parse_required::<PathBuf>(matches, name)?;
-    Eth2TestnetConfig::load(path.clone())
+    Eth2NetworkConfig::load(path.clone())
         .map_err(|e| format!("Unable to open testnet dir at {:?}: {}", path, e))
         .map(Some)
 }
@@ -28,9 +28,9 @@ pub fn parse_testnet_dir(
 pub fn parse_hardcoded_network(
     matches: &ArgMatches,
     name: &str,
-) -> Result<Option<Eth2TestnetConfig>, String> {
+) -> Result<Option<Eth2NetworkConfig>, String> {
     let network_name = parse_required::<String>(matches, name)?;
-    Eth2TestnetConfig::constant(network_name.as_str())
+    Eth2NetworkConfig::constant(network_name.as_str())
 }
 
 /// If `name` is in `matches`, parses the value as a path. Otherwise, attempts to find the user's