Make setting up BLS key transparant to validators

[gitferry]

BLS should be transparent to the validators so that no extra steps are needed to become a Babylon validator compared with other Cosmos-based chains. To achieve this, some changes are needed:

• move BLS key from priv_validator_key.json to a separate file priv_validator_bls.json (compatible with Horcrux)
• ensure the BLS key file is created atomically with priv_validator_key.json and remove create-bls-key cmd