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stating we should try to emulate `https://github.com/eth2-clients/eth2-testnets/tree/master/nimbus/testnet1` it looks outdated (?)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

account_manager/src/validator/exit.rs

@@ -7,7 +7,7 @@ use eth2::{
     BeaconNodeHttpClient, Url,
 };
 use eth2_keystore::Keystore;
-use eth2_testnet_config::Eth2TestnetConfig;
+use eth2_network_config::Eth2NetworkConfig;
 use safe_arith::SafeArith;
 use slot_clock::{SlotClock, SystemTimeSlotClock};
 use std::path::PathBuf;
@@ -99,7 +99,7 @@ async fn publish_voluntary_exit<E: EthSpec>(
     client: &BeaconNodeHttpClient,
     spec: &ChainSpec,
     stdin_inputs: bool,
-    testnet_config: &Eth2TestnetConfig,
+    testnet_config: &Eth2NetworkConfig,
 ) -> Result<(), String> {
     let genesis_data = get_geneisis_data(client).await?;
     let testnet_genesis_root = testnet_config