ipa-server-install with external CA: fix pkinit cert issuance
    
    ipa-server-install with external CA fails to issue pkinit certs.
    This happens because the installer calls
    krb = krbinstance.KrbInstance(fstore)
    then
    krb.enable_ssl()
    and in this code path self.config_pkinit is set to None, leading to a wrong
    code path.
    
    The fix initializes the required fields of the krbinstance before calling
    krb.enable_ssl.
    
    https://pagure.io/freeipa/issue/6921
    
    Reviewed-By: Stanislav Laznicka <slaznick@redhat.com>
    Reviewed-By: Abhijeet Kasurde <akasurde@redhat.com>