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.

Cargo.lock

@@ -4,6 +4,7 @@
 name = "account_manager"
 version = "0.0.1"
 dependencies = [
+ "account_utils",
  "bls",
  "clap",
  "clap_utils",
@@ -30,6 +31,18 @@ dependencies = [
  "web3",
 ]
 
+[[package]]
+name = "account_utils"
+version = "0.1.0"
+dependencies = [
+ "eth2_keystore",
+ "eth2_wallet",
+ "rand 0.7.3",
+ "serde",
+ "serde_derive",
+ "zeroize",
+]
+
 [[package]]
 name = "addr2line"
 version = "0.13.0"
@@ -5980,6 +5993,7 @@ dependencies = [
 name = "validator_client"
 version = "0.1.2"
 dependencies = [
+ "account_utils",
  "bls",
  "clap",
  "clap_utils",
@@ -5988,6 +6002,7 @@ dependencies = [
  "environment",
  "eth2_config",
  "eth2_interop_keypairs",
+ "eth2_keystore",
  "eth2_ssz",
  "eth2_ssz_derive",
  "exit-future",
@@ -6002,6 +6017,7 @@ dependencies = [
  "serde",
  "serde_derive",
  "serde_json",
+ "serde_yaml",
  "slashing_protection",
  "slog",
  "slog-async",