New Plugin Architecture: introduce command extensions dispatcher

The `CLICmdDispatcher` is responsible for loading and configuring all
plugins based on `CLICmdBase`. After registering the entry point to
the documented namespace (avocado.plugins.cli.cmd) the new command
should be visible just by running `$ avocado` or `$ avocado --help`.
Signed-off-by: NCleber Rosa <crosa@redhat.com>

avocado/core/app.py

@@ -20,6 +20,7 @@ import os
 
 from .log import configure as configure_log
 from .parser import Parser
+from .dispatcher import CLICmdDispatcher
 
 
 class AvocadoApp(object):
@@ -35,13 +36,12 @@ class AvocadoApp(object):
 
         configure_log()
         self.parser = Parser()
+        self.cli_cmd_dispatcher = CLICmdDispatcher()
         self.parser.start()
+        if self.cli_cmd_dispatcher.extensions:
+            self.cli_cmd_dispatcher.map_method('configure', self.parser)
+        self.parser.finish()
         self.ready = True
-        try:
-            self.parser.resume()
-            self.parser.finish()
-        except IOError:
-            self.ready = False
 
     def run(self):
         if self.ready:

avocado/core/dispatcher.py

+# This program is free software; you can redistribute it and/or modify
+# it under the terms of the GNU General Public License as published by
+# the Free Software Foundation; either version 2 of the License, or
+# (at your option) any later version.
+#
+# This program is distributed in the hope that it will be useful,
+# but WITHOUT ANY WARRANTY; without even the implied warranty of
+# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.
+#
+# See LICENSE for more details.
+#
+# Copyright: Red Hat Inc. 2015
+# Author: Cleber Rosa <cleber@redhat.com>
+
+"""Extensions/plugins dispatchers."""
+
+from stevedore import ExtensionManager
+
+
+class Dispatcher(ExtensionManager):
+
+    """
+    Base dispatcher for various extension types
+    """
+
+    def __init__(self, namespace):
+        self.load_failures = []
+        super(Dispatcher, self).__init__(namespace=namespace,
+                                         invoke_on_load=True,
+                                         on_load_failure_callback=self.store_load_failure)
+
+    @staticmethod
+    def store_load_failure(manager, entrypoint, exception):
+        manager.load_failures.append((entrypoint, exception))
+
+
+class CLICmdDispatcher(Dispatcher):
+
+    """
+    Calls extensions on configure/run
+
+    Automatically adds all the extension with entry points registered under
+    'avocado.plugins.cli.cmd'
+    """
+
+    def __init__(self):
+        super(CLICmdDispatcher, self).__init__('avocado.plugins.cli.cmd')

avocado/core/parser.py

@@ -70,12 +70,6 @@ class Parser(object):
             description='valid subcommands',
             help='subcommand help')
 
-    def resume(self):
-        """
-        Resume the parsing of arguments.
-
-        Side effect: update attribute `args` (the namespace).
-        """
         if tree.MULTIPLEX_CAPABLE:
             # Allow overriding multiplex variants by plugins args
             self.args.default_multiplex_tree = tree.TreeNode()