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/cli.rs

@@ -37,11 +37,19 @@ pub fn cli_app<'a, 'b>() -> App<'a, 'b> {
                        nodes using the same key. Automatically enabled unless `--strict` is specified",
         ))
         .arg(
-            Arg::with_name("strict")
-            .long("strict")
+            Arg::with_name("strict-lockfiles")
+            .long("strict-lockfiles")
             .help(
-                "If present, require that validator keypairs are unlocked and that auto-register \
-                is explicit before new validators are allowed to be used."
+                "If present, do not load validators that have are guarded by a lockfile. Note: for \
+                Eth2 mainnet, this flag will likely be removed and its behaviour will become default."
+            )
+        )
+        .arg(
+            Arg::with_name("disable-auto-discover")
+            .long("disable-auto-discover")
+            .help(
+                "If present, do not attempt to discover new validators in the validators-dir. Validators \
+                will need to be manually added to the validator_definitions.yml file."
             )
         )
         .arg(