ipatests: fix the disable_dnssec_validation method
    
    Bind configuration now includes 2 snippet config files, in
    /etc/named/ipa-ext.conf and /etc/named/ipa-options-ext.conf
    
    When a test needs to disable dnssec-validation, it needs to edit
    the snippet ipa-options-ext.conf instead of /etc/named.conf.
    
    This commit fixes the method tasks.disable_dnssec_validation so that it
    correctly updates the snippet.
    
    Fixes: https://pagure.io/freeipa/issue/8364
    Reviewed-By: Christian Heimes <cheimes@redhat.com>
    Reviewed-By: Alexander Bokovoy <abbra@users.noreply.github.com>