I found this bug when testing the GC patches. Previously, when SSSD was
started, but subdomains list was up-to-date, the ad_ctx was not
initialized for the subdomain.
I was also thinking whether we should re-initialize the domain-realm
mappings after sssd startup, the way we re-initialize kdcinfo files. I
don't think it's strictly necessary because if someone deletes a file in
/var/lib/sss/pubconf, he should keep the broken pieces, but perhaps we
should be at least aware..