ipa-kdb: use magic value to check if ipadb is used
    
    The certauth plugin is configured in /etc/krb5.conf independently form
    the database module. As a result the IPA certauth plugin can be added to
    the configuration without the IPA DAL driver. Since the IPA certauth
    plugin depends on the presence of the IPA DAL driver this patch adds a
    magic value at the beginning of struct ipadb_context which can be
    checked to see if the IPA DAL driver is properly initialized.
    
    Resolves https://pagure.io/freeipa/issue/7261
    
    Reviewed-By: Christian Heimes <cheimes@redhat.com>
    Reviewed-By: Christian Heimes <cheimes@redhat.com>