extra_iso: Include extra_files.json metadata

This should be on top level of the ISO, and list files added
specifically to the ISO. If there's anything inherited from one some
variant, the files will be listed in metadata in the variant directory.

JIRA: COMPOSE-3069
Signed-off-by: Lubomír Sedlář <lsedlar@redhat.com>

tests/helpers.py

@@ -149,7 +149,6 @@ class DummyCompose(object):
         self.conf = load_config(PKGSET_REPOS, **config)
         checks.validate(self.conf)
         self.paths = paths.Paths(self)
-        self._logger = mock.Mock()
         self.has_comps = True
         self.variants = {
             'Server': MockVariant(uid='Server', arches=['x86_64', 'amd64'],
@@ -162,7 +161,7 @@ class DummyCompose(object):
         self.all_variants = self.variants.copy()
 
         # for PhaseLoggerMixin
-        self._logger = mock.Mock()
+        self._logger = mock.Mock(name="compose._logger")
         self._logger.handlers = [mock.Mock()]
 
         self.log_info = mock.Mock()