plugins.variants: Fix destination

The `--system-wide` option's destination is "variants-skip-defaults" and
not "variants_skip_defaults" due which this option did not actually
work.
Signed-off-by: NLukáš Doktor <ldoktor@redhat.com>

avocado/core/varianter.py

@@ -385,7 +385,7 @@ class Varianter(object):
         self.default_params.clear()     # We don't need these anymore
         # FIXME: Backward compatibility params, to be removed when 36 LTS is
         # discontinued
-        if (not getattr(args, "variants_skip_defaults", False) and
+        if (not getattr(args, "variants-skip-defaults", False) and
                 hasattr(args, "default_avocado_params")):
             self._default_params.merge(args.default_avocado_params)
         return self._default_params