ipa: allow mpg group objects in apply_subdomain_homedir()
Since with algorithmic id-mapping SSSD automatically creates user
private groups for AD user with the help of magic private groups (mpg)
apply_subdomain_homedir() should be aware the in mpg domains a group
lookup might actually return a user object. Since the related sysdb
calls are clever and replace the objectcategory so that it matches the
original request type we have to check for the group category in the mpg
case as well. apply_subdomain_homedir() checks the uidNumber later as
well to make sure the object has the needed attributes for a user.
Related to https://pagure.io/SSSD/sssd/issue/3748
Reviewed-by: Jakub Hrozek <jhrozek@redhat.com>