plugins.multiplexer: rename filter keywords

Rename them from filter_out and filter_only to filter-out and filter-only,
respectively.

Motivation: consistent with original RFC and easier to the eyes, specially when
new filter keywords get added (filter-depth, filter-value, etc).

avocado/multiplexer.py

@@ -108,8 +108,8 @@ def multiplex(*args, **kwargs):
         # (not implemented here, so using [] as placeholders)
         result = [x + [y] for x in result for y in pool
                   if any_sibling(*(x + [y])) is False and
-                  filter_only(y.environment.get('filter_only', []), x + [y]) and
-                  filter_out(y.environment.get('filter_out', []), x + [y])]
+                  filter_only(y.environment.get('filter-only', []), x + [y]) and
+                  filter_out(y.environment.get('filter-out', []), x + [y])]
 
         # if a pool gets totally filtered out above, result will be empty
         if len(result) == 0: