The directory validator_keys will contain the deposit_data-TIMESTAMP.json and keystore-m_ID.json
files created by eth2.0-deposit-cli.
Use deposit_data-TIMESTAMP.json for your initial deposit. After that, it can be disposed of.
Use keystore-m_ID.json files to import your validator secret keys into the validator
instance of the client you are running. These files need to be secured when you are done
with the initial import.