Remove ra_db argument from CAInstance init
    
    The ra_db argument to CAInstance init is a constant so it can
    be removed. This constant corresponds to the default CertDB directory
    and since CertDB now passes passwords to its inner NSSDatabase instance
    we do need to care about having our own run_certutil() method.
    
    https://fedorahosted.org/freeipa/ticket/5695
    
    Reviewed-By: Christian Heimes <cheimes@redhat.com>
    Reviewed-By: Tomas Krizek <tkrizek@redhat.com>