tools: Validate TimeZone values in engine-config

Added validation for DefaultWindowsTimeZone and DefaultGeneralTimeZone
keys.

Bug-Url: https://bugzilla.redhat.com/show_bug.cgi?id=989396
Change-Id: I78c056128f1fcd607e9baa882e24544c99ad7cd2
Signed-off-by: Martin Betak <mbetak@redhat.com>