When new subdomains are discovered, the SSSD creates a file that
includes the domain-realm mappings. This file can in turn be included in
the krb5.conf using the includedir directive, such as:
includedir /var/lib/sss/pubconf/realm_mappings
The other part of the work is changing ipa-client such that the above
line is added to krb5.conf when the client is installed.