Updated validator config according to suggestions.

- Directory structure changed slightly
 - Uses a filter_map instead of a for loop.
 - All errors reading files does not prevent others from being read.
 - The accounts manager needs to generate files first, with the same structure.

validator_client/README.md

@@ -65,7 +65,6 @@ with `--datadir`.
 The configuration directory structure looks like:
 ```
 ~/.lighthouse-validator
-└── validators
     ├── 3cf4210d58ec
     │   └── private.key
     ├── 9b5d8b5be4e7