avocado: Add a mechanism to skip notifying broken plugins on stderr

Implement a feature to skip notifying broken plugins on
stderr. Let's say a user has a broken plugin that can't
be fixed right now, and he/she doesn't want to be bothered
by having it notified in stderr all the time avocado is
executed, add a configuration option that allows you to
selectively ignore plugins to display errors in stderr.

The reason why the feature is configurable is that we
want users to be notified when a new plugin is present
and for some reason is also broken, this finer grained
control is more useful to users IMHO.
Signed-off-by: NLucas Meneghel Rodrigues <lmr@redhat.com>

avocado/core/plugins/builtin.py

@@ -19,6 +19,7 @@ import logging
 from importlib import import_module
 
 from avocado.core.plugins.plugin import Plugin
+from avocado.core.settings import settings
 
 
 log = logging.getLogger("avocado.app")
@@ -46,12 +47,16 @@ def load_builtins():
     :return: a list of plugin classes, ordered by `priority`.
     """
     plugins = []
+    skip_notify = settings.get_value(section='plugins',
+                                     key='skip_broken_plugin_notification',
+                                     key_type=list)
     for module in Builtins:
         try:
             plugin_mod = import_module(module)
         except Exception as err:
             name = str(module)
             reason = '%s %s' % (str(err.__class__.__name__), err)
+            if name not in skip_notify:
                 log.error('Error loading %s -> %s', name, reason)
             ErrorsLoading.append((name, reason))
             continue

etc/avocado/avocado.conf

@@ -46,3 +46,11 @@ port = 9405
 username =
 # If authentication is set, pass password
 password =
+
+[plugins]
+# Suppress notification about broken plugins in the app standard error.
+# Add the name of each broken plugin you want to suppress the notification
+# in the list. The names can be easily seen from the stderr messages. Example:
+# avocado.core.plugins.htmlresult  ImportError No module named pystache
+# add 'avocado.core.plugins.htmlresult' as an element of the list below.
+skip_broken_plugin_notification = []