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.

validator_client/src/config.rs

@@ -23,10 +23,10 @@ pub struct Config {
     /// If true, the validator client will still poll for duties and produce blocks even if the
     /// beacon node is not synced at startup.
     pub allow_unsynced_beacon_node: bool,
-    /// If true, we will be strict about concurrency and validator registration.
-    pub strict: bool,
-    /// If true, register new validator keys with the slashing protection database.
-    pub auto_register: bool,
+    /// If true, refuse to unlock a keypair that is guarded by a lockfile.
+    pub strict_lockfiles: bool,
+    /// If true, don't scan the validators dir for new keystores.
+    pub disable_auto_discover: bool,
 }
 
 impl Default for Config {
@@ -43,8 +43,8 @@ impl Default for Config {
             secrets_dir,
             http_server: DEFAULT_HTTP_SERVER.to_string(),
             allow_unsynced_beacon_node: false,
-            auto_register: false,
-            strict: false,
+            strict_lockfiles: false,
+            disable_auto_discover: false,
         }
     }
 }
@@ -73,13 +73,8 @@ impl Config {
         }
 
         config.allow_unsynced_beacon_node = cli_args.is_present("allow-unsynced");
-        config.auto_register = cli_args.is_present("auto-register");
-        config.strict = cli_args.is_present("strict");
-
-        if !config.strict {
-            // Do not require an explicit `--auto-register` if `--strict` is disabled.
-            config.auto_register = true
-        }
+        config.strict_lockfiles = cli_args.is_present("strict-lockfiles");
+        config.disable_auto_discover = cli_args.is_present("disable-auto-discover");
 
         if let Some(secrets_dir) = parse_optional(cli_args, "secrets-dir")? {
             config.secrets_dir = secrets_dir;