Plugins: use the plugin type instead of entry point namespace

The setuptools entry point namespace, currently being show in the
output of the plugin listing, is not that relevant to users.

Let's use the plugin types (as documented in the section about the
fully qualified name of plugins), which is more relevant to Avocado
users.
Signed-off-by: NCleber Rosa <crosa@redhat.com>

avocado/plugins/plugins.py

@@ -42,13 +42,13 @@ class Plugins(CLICmd):
         log = logging.getLogger("avocado.app")
         plugin_types = [
             (dispatcher.CLICmdDispatcher(),
-             'Plugins that add new commands (avocado.plugins.cli.cmd):'),
+             'Plugins that add new commands (cli.cmd):'),
             (dispatcher.CLIDispatcher(),
-             'Plugins that add new options to commands (avocado.plugins.cli):'),
+             'Plugins that add new options to commands (cli):'),
             (dispatcher.JobPrePostDispatcher(),
-             'Plugins that run before/after the execution of jobs (avocado.plugins.job.prepost):'),
+             'Plugins that run before/after the execution of jobs (job.prepost):'),
             (dispatcher.ResultDispatcher(),
-             'Plugins that generate job result in different formats (avocado.plugins.result):')
+             'Plugins that generate job result in different formats (result):')
         ]
         for plugins_active, msg in plugin_types:
             log.info(msg)