Define %{_module_context} and %{disttag}
No reason to hide module context from the consumers, plus it's useful to
keep it in the DISTTAG tag (not to be confused with the %{dist} tag) for
tracking.
The %{disttag} feature will do nothing until RPM supports it. This is
planned in 4.14.2.
See https://bugzilla.redhat.com/show_bug.cgi?id=1596192.
Signed-off-by: Petr Šabata <contyk@redhat.com>