Fix re-key after importing existing private key
    
    If the key_gen size and algorithm are not set then set it
    with the current key. This is important when generating a
    certificate using a pre-existing private key.
    
    Previously a resubmit would cause certmonger to generate a new
    2048-bit key regardless of the previous key size.
    
    https://pagure.io/certmonger/issue/124