check valid session keys on Polkadot node versus on chain setKeys value

[branciard]

Valid session keys on the validator node keystore is import as this recent slashing problem show :

Adding an optional variable POLKADOT_SESSION_KEY_TO_CHECK
To be able at any time to check if the polkadot node as a valid session key by calling author_hasSessionKeys API when starting the node is a good security measure.