Instead of copying a block of code that checks whether domain is a subdomain
and uses only name of FQDN as appropriate, wrap the logic into a function.

Instead of using printf-like functions directly, provide two wrappers
that would encapsulate formatting the fully-qualified names. No
functional change is present in this patch.

Fixes https://fedorahosted.org/sssd/ticket/1766

A sysdb contains now multiple domains, but the mpg property is a
property of a specific domain not of the underlying database.