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stating we should try to emulate `https://github.com/eth2-clients/eth2-testnets/tree/master/nimbus/testnet1` it looks outdated (?)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

Cargo.lock

@@ -14,9 +14,9 @@ dependencies = [
  "environment",
  "eth2",
  "eth2_keystore",
+ "eth2_network_config",
  "eth2_ssz",
  "eth2_ssz_derive",
- "eth2_testnet_config",
  "eth2_wallet",
  "eth2_wallet_manager",
  "futures 0.3.8",
@@ -592,8 +592,8 @@ dependencies = [
  "environment",
  "eth2_config",
  "eth2_libp2p",
+ "eth2_network_config",
  "eth2_ssz",
- "eth2_testnet_config",
  "exit-future",
  "futures 0.3.8",
  "genesis",
@@ -816,8 +816,8 @@ dependencies = [
  "beacon_node",
  "clap",
  "eth2_libp2p",
+ "eth2_network_config",
  "eth2_ssz",
- "eth2_testnet_config",
  "futures 0.3.8",
  "hex",
  "log 0.4.11",
@@ -1048,8 +1048,8 @@ version = "0.1.0"
 dependencies = [
  "clap",
  "dirs 3.0.1",
+ "eth2_network_config",
  "eth2_ssz",
- "eth2_testnet_config",
  "hex",
  "types",
 ]
@@ -1604,7 +1604,7 @@ dependencies = [
  "clap",
  "clap_utils",
  "dirs 3.0.1",
- "eth2_testnet_config",
+ "eth2_network_config",
 ]
 
 [[package]]
@@ -1851,7 +1851,7 @@ version = "0.1.2"
 dependencies = [
  "ctrlc",
  "eth2_config",
- "eth2_testnet_config",
+ "eth2_network_config",
  "exit-future",
  "futures 0.3.8",
  "logging",
@@ -2062,6 +2062,20 @@ dependencies = [
  "void",
 ]
 
+[[package]]
+name = "eth2_network_config"
+version = "0.2.0"
+dependencies = [
+ "enr",
+ "eth2_config",
+ "eth2_ssz",
+ "serde",
+ "serde_yaml",
+ "tempdir",
+ "types",
+ "zip",
+]
+
 [[package]]
 name = "eth2_ssz"
 version = "0.1.2"
@@ -2094,20 +2108,6 @@ dependencies = [
  "typenum",
 ]
 
-[[package]]
-name = "eth2_testnet_config"
-version = "0.2.0"
-dependencies = [
- "enr",
- "eth2_config",
- "eth2_ssz",
- "serde",
- "serde_yaml",
- "tempdir",
- "types",
- "zip",
-]
-
 [[package]]
 name = "eth2_wallet"
 version = "0.1.0"
@@ -3483,8 +3483,8 @@ dependencies = [
  "environment",
  "eth2_keystore",
  "eth2_libp2p",
+ "eth2_network_config",
  "eth2_ssz",
- "eth2_testnet_config",
  "futures 0.3.8",
  "genesis",
  "hex",
@@ -3877,7 +3877,7 @@ dependencies = [
  "directory",
  "env_logger 0.8.2",
  "environment",
- "eth2_testnet_config",
+ "eth2_network_config",
  "futures 0.3.8",
  "lighthouse_version",
  "logging",