build: list only buildable templates for --template-only

TripleO uses --template-only to generate Dockerfiles and then use
buildah to create container images.

But as Kolla generates all files then they need to have a way to exclude
not buildable ones. https://review.opendev.org/#/c/699024 is one of such
attempts.

But if we remove not buildable Dockerfiles then they get what they can
build.

Change-Id: I7a92b762f6a783f92ebca61e5595e1422a92df4f

kolla/image/build.py

@@ -1433,6 +1433,16 @@ def run_build():
     kolla.create_dockerfiles()
 
     if conf.template_only:
+        kolla.build_image_list()
+        kolla.find_parents()
+        kolla.filter_images()
+
+        for image in kolla.images:
+            if image.status == STATUS_MATCHED:
+                continue
+
+            shutil.rmtree(image.path)
+
         LOG.info('Dockerfiles are generated in %s', kolla.working_dir)
         return