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.

testing/simulator/src/eth1_sim.rs

@@ -130,7 +130,7 @@ pub fn run_eth1_sim(matches: &ArgMatches) -> Result<(), String> {
             network
                 .add_validator_client(
                     ValidatorConfig {
-                        auto_register: true,
+                        disable_auto_discover: false,
                         ..ValidatorConfig::default()
                     },
                     i,

testing/simulator/src/no_eth1_sim.rs

@@ -104,7 +104,7 @@ pub fn run_no_eth1_sim(matches: &ArgMatches) -> Result<(), String> {
                 network
                     .add_validator_client(
                         ValidatorConfig {
-                            auto_register: true,
+                            disable_auto_discover: false,
                             ..ValidatorConfig::default()
                         },
                         i,