Fix naming of validators in CLI (#1332)

## Issue Addressed

NA

## Proposed Changes

- Adds a `lighthouse account validator list` command, to list all known validators.
- Fixes the validator names; previously they were using a full path (e.g., `"/home/paul/.lighthouse/validators/0x8ce25415d078bdc83133758604578ba51707a55965eeca8982f44695db7432d6ff1c23529020a971faa68ab60baf3118"` but now we only use the final directory name (e.g., `0x8ce25415d078bdc83133758604578ba51707a55965eeca8982f44695db7432d6ff1c23529020a971faa68ab60baf3118`).

account_manager/src/validator/list.rs

@@ -0,0 +1,30 @@
+use crate::VALIDATOR_DIR_FLAG;
+use clap::{App, ArgMatches};
+use std::path::PathBuf;
+use validator_dir::Manager as ValidatorManager;
+
+pub const CMD: &str = "list";
+
+pub fn cli_app<'a, 'b>() -> App<'a, 'b> {
+    App::new(CMD).about("Lists the names of all validators.")
+}
+
+pub fn cli_run(matches: &ArgMatches<'_>) -> Result<(), String> {
+    let data_dir = clap_utils::parse_path_with_default_in_home_dir(
+        matches,
+        VALIDATOR_DIR_FLAG,
+        PathBuf::new().join(".lighthouse").join("validators"),
+    )?;
+
+    let mgr = ValidatorManager::open(&data_dir)
+        .map_err(|e| format!("Unable to read --{}: {:?}", VALIDATOR_DIR_FLAG, e))?;
+
+    for (name, _path) in mgr
+        .directory_names()
+        .map_err(|e| format!("Unable to list wallets: {:?}", e))?
+    {
+        println!("{}", name)
+    }
+
+    Ok(())
+}