Allow specifying signing algorithm of the IPA CA cert in ipa-ca-install
    
    The --ca-signing-algorithm option is available in ipa-server-install, make
    it available in ipa-ca-install as well.
    
    https://fedorahosted.org/freeipa/ticket/4447
    
    Reviewed-By: Petr Vobornik <pvoborni@redhat.com>