fix: vault variant auth method

All vault variants are currently failing by default.

Users have to add VAULT_AUTH_METHOD: "jwt" to make them work. Furthermore it is hard to guess as the variable is only in https://gitlab.com/to-be-continuous/tools/vault-secrets-provider

The reason is than the the default VAULT_AUTH_METHOD check if VAULT_ROLE_ID and VAULT_SECRET_ID are not empty. If so it uses approle otherwise it will try VAULT_JWT_TOKEN

But with the current setup those variables are no defaulting to an empty string "" but to $VAULT_ROLE_ID and $VAULT_SECRET_ID respectively