Introduce validator definition file for VC (#1357)

## Issue Addressed

NA

## Proposed Changes

- Introduces the `valdiator_definitions.yml` file which serves as an explicit list of validators that should be run by the validator client.
  - Removes `--strict` flag, split into `--strict-lockfiles` and `--disable-auto-discover`  
  - Adds a "Validator Management" page to the book.
- Adds the `common/account_utils` crate which contains some logic that was starting to duplicate across the codebase.

The new docs for this feature are the best description of it (apart from the code, I guess): 9cb87e93ce/book/src/validator-management.md

## API Changes

This change should be transparent for *most* existing users. If the `valdiator_definitions.yml` doesn't exist then it will be automatically generated using a method that will detect all the validators in their `validators_dir`.

Users will have issues if they are:

1. Using `--strict`.
1. Have keystores in their `~/.lighthouse/validators` directory that weren't being detected by the current keystore discovery method.

For users with (1), the VC will refuse to start because the `--strict` flag has been removed. They will be forced to review `--help` and choose an equivalent flag.

For users with (2), this seems fairly unlikely and since we're only in testnets there's no *real* value on the line here. I'm happy to take the risk, it would be a different case for mainnet.

## Additional Info

This PR adds functionality we will need for #1347.

## TODO

- [x] Reconsider flags
- [x] Move doc into a more reasonable chapter.
- [x] Check for compile warnings.

account_manager/src/common.rs

@@ -1,25 +1,7 @@
 use clap::ArgMatches;
-use eth2_wallet::PlainText;
-use rand::{distributions::Alphanumeric, Rng};
 use std::fs::create_dir_all;
 use std::path::{Path, PathBuf};
 
-/// The `Alphanumeric` crate only generates a-z, A-Z, 0-9, therefore it has a range of 62
-/// characters.
-///
-/// 62**48 is greater than 255**32, therefore this password has more bits of entropy than a byte
-/// array of length 32.
-const DEFAULT_PASSWORD_LEN: usize = 48;
-
-pub fn random_password() -> PlainText {
-    rand::thread_rng()
-        .sample_iter(&Alphanumeric)
-        .take(DEFAULT_PASSWORD_LEN)
-        .collect::<String>()
-        .into_bytes()
-        .into()
-}
-
 pub fn ensure_dir_exists<P: AsRef<Path>>(path: P) -> Result<(), String> {
     let path = path.as_ref();
 
@@ -37,56 +19,3 @@ pub fn base_wallet_dir(matches: &ArgMatches, arg: &'static str) -> Result<PathBu
         PathBuf::new().join(".lighthouse").join("wallets"),
     )
 }
-
-/// Remove any number of newline or carriage returns from the end of a vector of bytes.
-pub fn strip_off_newlines(mut bytes: Vec<u8>) -> Vec<u8> {
-    let mut strip_off = 0;
-    for (i, byte) in bytes.iter().rev().enumerate() {
-        if *byte == b'\n' || *byte == b'\r' {
-            strip_off = i + 1;
-        } else {
-            break;
-        }
-    }
-    bytes.truncate(bytes.len() - strip_off);
-    bytes
-}
-
-#[cfg(test)]
-mod test {
-    use super::strip_off_newlines;
-
-    #[test]
-    fn test_strip_off() {
-        let expected = "hello world".as_bytes().to_vec();
-
-        assert_eq!(
-            strip_off_newlines("hello world\n".as_bytes().to_vec()),
-            expected
-        );
-        assert_eq!(
-            strip_off_newlines("hello world\n\n\n\n".as_bytes().to_vec()),
-            expected
-        );
-        assert_eq!(
-            strip_off_newlines("hello world\r".as_bytes().to_vec()),
-            expected
-        );
-        assert_eq!(
-            strip_off_newlines("hello world\r\r\r\r\r".as_bytes().to_vec()),
-            expected
-        );
-        assert_eq!(
-            strip_off_newlines("hello world\r\n".as_bytes().to_vec()),
-            expected
-        );
-        assert_eq!(
-            strip_off_newlines("hello world\r\n\r\n".as_bytes().to_vec()),
-            expected
-        );
-        assert_eq!(
-            strip_off_newlines("hello world".as_bytes().to_vec()),
-            expected
-        );
-    }
-}

account_manager/src/validator/create.rs

@@ -1,7 +1,5 @@
-use crate::{
-    common::{ensure_dir_exists, random_password, strip_off_newlines},
-    SECRETS_DIR_FLAG, VALIDATOR_DIR_FLAG,
-};
+use crate::{common::ensure_dir_exists, SECRETS_DIR_FLAG, VALIDATOR_DIR_FLAG};
+use account_utils::{random_password, strip_off_newlines};
 use clap::{App, Arg, ArgMatches};
 use environment::Environment;
 use eth2_wallet::PlainText;

account_manager/src/wallet/create.rs

@@ -1,7 +1,5 @@
-use crate::{
-    common::{random_password, strip_off_newlines},
-    BASE_DIR_FLAG,
-};
+use crate::BASE_DIR_FLAG;
+use account_utils::{random_password, strip_off_newlines};
 use clap::{App, Arg, ArgMatches};
 use eth2_wallet::{
     bip39::{Language, Mnemonic, MnemonicType},