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stating we should try to emulate `https://github.com/eth2-clients/eth2-testnets/tree/master/nimbus/testnet1` it looks outdated (?)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

beacon_node/eth2_libp2p/src/config.rs

@@ -1,7 +1,7 @@
 use crate::types::{GossipKind, MessageData};
 use crate::{Enr, PeerIdSerialized};
 use directory::{
-    DEFAULT_BEACON_NODE_DIR, DEFAULT_HARDCODED_TESTNET, DEFAULT_NETWORK_DIR, DEFAULT_ROOT_DIR,
+    DEFAULT_BEACON_NODE_DIR, DEFAULT_HARDCODED_NETWORK, DEFAULT_NETWORK_DIR, DEFAULT_ROOT_DIR,
 };
 use discv5::{Discv5Config, Discv5ConfigBuilder};
 use libp2p::gossipsub::{
@@ -104,7 +104,7 @@ impl Default for Config {
         let network_dir = dirs::home_dir()
             .unwrap_or_else(|| PathBuf::from("."))
             .join(DEFAULT_ROOT_DIR)
-            .join(DEFAULT_HARDCODED_TESTNET)
+            .join(DEFAULT_HARDCODED_NETWORK)
             .join(DEFAULT_BEACON_NODE_DIR)
             .join(DEFAULT_NETWORK_DIR);