[checks] Don't always require jigdo

If the configuration specifically requests no jigdos, there is no point
in checking for the binary existence.

This is not 100% reliable. The jigdo option defaults to True, so if the
option is not specified the binary is required even if there are no
images configured.

Signed-off-by: Lubomír Sedlář <lsedlar@redhat.com>

tests/data/dummy-pungi.conf

@@ -109,3 +109,5 @@ createiso_skip = [
         'src': True
     }),
 ]
+
+create_jigdo = False