Add IPA specific vars to ipaca_default.ini
    
    Common settings like "pki_*_signing_key_algorithm" now use an IPA
    specific template variable. The approach makes it easier to change all
    signing parameters to use a different algorithm.
    
    Fixes: https://pagure.io/freeipa/issue/5608
    Signed-off-by: Christian Heimes <cheimes@redhat.com>
    Reviewed-By: Fraser Tweedale <ftweedal@redhat.com>
    Reviewed-By: Alexander Bokovoy <abokovoy@redhat.com>