Clustering and SSL are pretty much orthogonal, its only about configuring your Apache and Undertow in WildFly to use SSL.
For undertow you will have to do something similar to configuration as here in the first post: Wildfly Undertow SSL performance issue or look at the docs here Undertow (web) subsystem configuration - WildFly 8 - Project Documentation Editor
For Apache look at the appropriate documentation for the module you are using, i.e. mod_jk, mod_proxy or mod_cluster (mod_cluster Documentation)...
I would recommend to set https only on the httpd side and use ajp for communication between httpd and wildfly. This will make your environment very simple.