output: Support Paginator.flush

While debugging Avocado I noticed some crashes when attaching debugger.
It was caused by the debugger which force-flushes the STDOUT on some
occasions, which is usually supported, but is not supported by our
Paginator class. This commit adds the flush method which flushes the
output pipe (unless it's already closed)
Signed-off-by: NLukáš Doktor <ldoktor@redhat.com>

avocado/core/output.py

@@ -561,6 +561,10 @@ class Paginator(object):
         except Exception:
             pass
 
+    def flush(self):
+        if not self.pipe.closed:
+            self.pipe.flush()
+
 
 def add_log_handler(logger, klass=logging.StreamHandler, stream=sys.stdout,
                     level=logging.INFO, fmt='%(name)s: %(message)s'):