output: Define LOG_UI and LOG_JOB

On many places we are directly getting "avocado.app" and "avocado.test"
loggers and interacting with them. This can become an issue when we try
to replace or modify our logging streams in the future as well as it
requires some hidden knowledge. Let's define LOG_UI and LOG_JOB in
`avocado.core.output` to be available for Avocado core (and silently
also for plugin) developers.
Signed-off-by: NLukáš Doktor <ldoktor@redhat.com>

avocado/core/output.py

@@ -32,6 +32,11 @@ else:
     NULL_HANDLER = logutils.NullHandler
 
 
+#: Pre-defined Avocado human UI logger
+LOG_UI = logging.getLogger("avocado.app")
+#: Pre-defined Avocado job/test logger
+LOG_JOB = logging.getLogger("avocado.test")
+
 #: Builtin special keywords to enable set of logging streams
 BUILTIN_STREAMS = {'app': 'application output',
                    'test': 'test output',

docs/source/Plugins.rst

@@ -44,6 +44,7 @@ Code example
 Let's say you want to write a plugin that adds a new subcommand to the test
 runner, ``hello``. This is how you'd do it::
 
+    from avocado.core.output import LOG_JOB
     from avocado.core.plugin_interfaces import CLICmd
 
 
@@ -53,13 +54,17 @@ runner, ``hello``. This is how you'd do it::
         description = 'The classical Hello World! plugin example.'
 
         def run(self, args):
-            print(self.description)
+            LOG_JOB.info(self.description)
 
 As you can see, this plugins inherits from :class:`avocado.core.plugin_interfaces.CLICmd`.
 This specific base class allows for the creation of new commands for the Avocado
 CLI tool. The only mandatory method to be implemented is :func:`run
 <avocado.core.plugin_interfaces.CLICmd.run>` and it's the plugin main entry point.
-In this code example it will simply print the plugin's description.
+
+This plugin uses :py:data:`avocado.core.output.LOG_JOB` to produce the hello
+world output in the Job log. One can also use
+:py:data:`avocado.core.output.LOG_UI` to produce output in the human readable
+output.
 
 Registering Plugins
 ~~~~~~~~~~~~~~~~~~~