ipatests: Add Unattended option to external ca task
    
    After resolving https://pagure.io/freeipa/issue/7930
    an unattended option alongsede with -r 'REALM' option
    needs to be passed to tasks which lacks ntp options.
    
    Reviewed-By: Rob Crittenden <rcritten@redhat.com>
    Reviewed-By: Oleg Kozlov <okozlov@redhat.com>