Import modules found in the local repositories to MBS database for Offline local builds.

This is the first PR in many for Offline local builds. This PR:

- Adds --offline flag to build_module_locally mbs-manager command to enable
  offline local builds.
- If this flag is used, new `import_builds_from_local_dnf_repos` method is
  called which uses DNF API to get all the available installable modulemd
  files and imports each module into MBS local SQLite database.
- It also adds fake "platform:stream" module based on the /etc/os-release,
  so the buildrequirements of the imported modules are satisfied.

The idea here is that in upcoming commits, I will create LocalResolver
which will be similar to DBResolver with some extra rules to resolve
local module builds. This new LocalResolver will still be based on
the models.ModuleBuild methods and therefore we need the modules
imported in database.

tests/test_utils/test_utils.py

@@ -1225,3 +1225,57 @@ class TestLocalBuilds:
             assert len(local_modules) == 1
             assert local_modules[0].koji_tag.endswith(
                 "/module-platform-f28-3/results")
+
+
+class TestOfflineLocalBuilds:
+
+    def setup_method(self):
+        clean_database()
+
+    def teardown_method(self):
+        clean_database()
+
+    def test_import_fake_base_module(self):
+        module_build_service.utils.import_fake_base_module("platform:foo:1:000000")
+        module_build = models.ModuleBuild.get_build_from_nsvc(
+            db.session, "platform", "foo", 1, "000000")
+        assert module_build
+
+        mmd = module_build.mmd()
+        xmd = glib.from_variant_dict(mmd.get_xmd())
+        assert xmd == {
+            'mbs': {
+                'buildrequires': {},
+                'commit': 'ref_000000',
+                'koji_tag': 'local_build',
+                'mse': 'true',
+                'requires': {}}}
+
+        profiles = mmd.get_profiles()
+        assert set(profiles.keys()) == set(["buildroot", "srpm-buildroot"])
+
+    @patch("module_build_service.utils.general.open", create=True)
+    def test_import_builds_from_local_dnf_repos(self, patched_open):
+        pytest.importorskip("dnf")
+
+        with patch("dnf.Base") as dnf_base:
+            repo = mock.MagicMock()
+            with open(path.join(BASE_DIR, '..', 'staged_data', 'formatted_testmodule.yaml')) as f:
+                repo.get_metadata_content.return_value = f.read()
+            base = dnf_base.return_value
+            base.repos = {"reponame": repo}
+
+            patched_open.return_value = mock.mock_open(
+                read_data="FOO=bar\nPLATFORM_ID=platform:x\n").return_value
+
+            module_build_service.utils.import_builds_from_local_dnf_repos()
+
+            base.read_all_repos.assert_called_once()
+            repo.load.assert_called_once()
+            repo.get_metadata_content.assert_called_once_with("modules")
+
+            module_build = models.ModuleBuild.get_build_from_nsvc(
+                db.session, "testmodule", "master", 20180205135154, "9c690d0e")
+            assert module_build
+            module_build = models.ModuleBuild.get_build_from_nsvc(
+                db.session, "platform", "x", 1, "000000")