Improve account manager CLI (#1404)

## Proposed Changes

Fixes some sharp edges on the new `lighthouse account validator list` command, and the account manager CLI.

* Validator names/keys are always printed in the same order due to the use of a sorted `BTreeMap`
* The `validator list` subcommand now respects the `--validator-dir` flag, instead of always looking in `~/.lighthouse/validators`
* The `--help` now shows a description for the `wallet` subcommand instead of just `TODO`

common/validator_dir/src/manager.rs

@@ -2,7 +2,7 @@ use crate::{Error as ValidatorDirError, ValidatorDir};
 use bls::Keypair;
 use rayon::prelude::*;
 use slog::{info, warn, Logger};
-use std::collections::HashMap;
+use std::collections::BTreeMap;
 use std::fs::read_dir;
 use std::io;
 use std::iter::FromIterator;
@@ -163,13 +163,13 @@ impl Manager {
     /// ## Errors
     ///
     /// Returns an error if a directory is unable to be read.
-    pub fn directory_names(&self) -> Result<HashMap<String, PathBuf>, Error> {
-        Ok(HashMap::from_iter(self.iter_dir()?.into_iter().filter_map(
-            |path| {
+    pub fn directory_names(&self) -> Result<BTreeMap<String, PathBuf>, Error> {
+        Ok(BTreeMap::from_iter(
+            self.iter_dir()?.into_iter().filter_map(|path| {
                 path.file_name()
                     .and_then(|os_string| os_string.to_str().map(|s| s.to_string()))
                     .map(|filename| (filename, path))
-            },
-        )))
+            }),
+        ))
     }
 }