Validator dir creation (#1746)

## Issue Addressed

Resolves #1744

## Proposed Changes

- Add `directory::ensure_dir_exists` to the `ValidatorDefinition::open_or_create` method 
- As @pawanjay176 suggested, making the `--validator-dir` non-global so users are forced to include the flag after the `validator` subcommand. Current behavior seems to be ignoring the flag if it comes after something like `validator import`

## Additional Info
N/A

account_manager/src/validator/mod.rs

@@ -26,7 +26,6 @@ pub fn cli_app<'a, 'b>() -> App<'a, 'b> {
                     Defaults to ~/.lighthouse/{testnet}/validators",
                 )
                 .takes_value(true)
-                .global(true)
                 .conflicts_with("datadir"),
         )
         .subcommand(create::cli_app())