avocado/result.py: remove confusing and unused 'console_plugin' attribute

Signed-off-by: Cleber Rosa <crosa@redhat.com>
result plugins: replace the set_output_option() method

For a simpler command_line_arg_name attribute. This way, there's no
need to define a method, simply define the attribute value. It's
consistent with the attributes (enabled, etc) defined in the Plugin
class.

Signed-off-by: Cleber Rosa <crosa@redhat.com>
result plugins: replace the set_output() method

With a simpler (and already existing) attribute. Having a method
suggests that it's going to be called multiple times, which is not
the case.

Signed-off-by: Cleber Rosa <crosa@redhat.com>
Merge pull request #206 from clebergnu/simplify_result_plugins

Simplify result plugins