frontend: Enable edit of time zone for Linux OS

Re-enabled editing of time zones for all OS types and added an info icon
describing the recommended usage.

Change-Id: I0e6106c903f307a45b31ed3ead0b66611d53d419
Bug-Url: https://bugzilla.redhat.com/show_bug.cgi?id=1014326
Signed-off-by: Martin Betak <mbetak@redhat.com>