future/settings: Migrate list to use the new future/settings API

As part of the effort to remove default values scattered around the
code, this change only migrates the list plugin options to use the new
module. This will associate the 'references' to 'list.references' on
configuration file.
Signed-off-by: NBeraldo Leal <bleal@redhat.com>

avocado/plugins/list.py

@@ -20,6 +20,7 @@ from avocado.core import test
 from avocado.core import tags
 from avocado.core import parser_common_args
 from avocado.core.output import LOG_UI
+from avocado.core.future.settings import settings
 from avocado.core.plugin_interfaces import CLICmd
 from avocado.utils import astring
 
@@ -122,7 +123,7 @@ class TestLister:
 
     def _list(self):
         self._extra_listing()
-        test_suite = self._get_test_suite(self.args.get('references', []))
+        test_suite = self._get_test_suite(self.args.get('list.references'))
         if self.args.get('filter_by_tags', False):
             test_suite = tags.filter_test_tags(
                 test_suite,
@@ -157,15 +158,20 @@ class List(CLICmd):
         :type parser: :class:`avocado.core.parser.ArgumentParser`
         """
         parser = super(List, self).configure(parser)
-        parser.add_argument('references', type=str, default=[], nargs='*',
-                            help="List of test references (aliases or paths). "
-                            "If empty, avocado will list tests on "
-                            "the configured test source, "
-                            "(see 'avocado config --datadir') Also, "
-                            "if there are other test loader plugins "
-                            "active, tests from those plugins might "
-                            "also show up (behavior may vary among "
-                            "plugins)")
+        help_msg = ('List of test references (aliases or paths). If empty, '
+                    'Avocado will list tests on the configured test source, '
+                    '(see "avocado config --datadir") Also, if there are '
+                    'other test loader plugins active, tests from those '
+                    'plugins might also show up (behavior may vary among '
+                    'plugins)')
+        settings.register_option(section='list',
+                                 key='references',
+                                 default=[],
+                                 nargs='*',
+                                 key_type=list,
+                                 help_msg=help_msg,
+                                 parser=parser,
+                                 positional_arg=True)
         loader.add_loader_options(parser)
         parser_common_args.add_tag_filter_args(parser)