You can use the Default Tomcat Realm to do the Authentication for Slide as follows:
You still need to set the path to Domain.xml explicitely in a slide.properties file. I located mine in Tomcat's .\common\classes and used forward slashes in the path instead of escaped backslashes.
I also put the Domain.xml there instead of .\webapps\slide, however that may not be necessary since we aren't useing SlideRealm.
If you do put Domain.xml someplace else and refer to it in slide.properties, make sure you don't have it also in webapps/slide, some classes apparently look there first as I found it wasn't working until I removed/renamed .\webapps\slide\Domain.xml
Uncommenting the <security-constraint> element in both slide\WEB-INF\web.xml and slide-admin\WEB-INF\web.xml is the same as when using the SlideRealm.
Caveats at least in my trial and error approach. I set the users, roles and groups in Tomcat to mirror those in Domain.xml but I am not sure that is entirely necessary to have all users, roles and groups defined in both places...I did and it works, so it isn't a definitive statement that it is needed.