frontend: Correct default time-zone rendering

TimeZoneModel was returning incorrect display value in case the
configured default was invalid. Now it returns display value
corresponding to ultimate fallback for given TimeZoneType.

Bug-Url: https://bugzilla.redhat.com/show_bug.cgi?id=988259
Change-Id: Ic7be050a48c78ffdf2028ea5acfd0123bf4502a7
Signed-off-by: Martin Betak <mbetak@redhat.com>