UTIL: Add a is_domain_mpg shorthand
Instead of looking into the domain structure directly, add a
sss_domain_is_mpg() function. This will make sense when we add a third
state instead of the boolean that will also be mpg-like.
Related:
https://pagure.io/SSSD/sssd/issue/3822
Reviewed-by: Michal Židek <mzidek@redhat.com>