Make flake8 config compatible with latest version

New ignored errors/warning:
- W504: line break after binary operator
- E741: do not use variables named ‘l’, ‘O’, or ‘I’

Fixed issues:
- E117: over-indented

This was tested locally with flake8==3.7.9 with Python 3.8.

The currently used version of flake8 (2.6.2) is not compatible with
Python 3.8. Unfortunately the dependency is coming in via
diskimage-builder>=2.0 -> hacking<1.2.0,>=1.1.0 -> flake8<2.7.0,>=2.6.0

Change-Id: Idf8b6033232fdce91cb37c8396dab6cc27c75968

nodepool/builder.py

@@ -82,7 +82,7 @@ class DibImageFile(object):
             if image_file.is_file():
                 image = DibImageFile.from_path(image_file)
                 if image.image_id == image_id:
-                        images.append(image)
+                    images.append(image)
         return images
 
     @staticmethod

tox.ini

@@ -63,6 +63,6 @@ commands = stestr --test-path ./nodepool/tests/functional/openshift run --no-sub
 [flake8]
 # These are ignored intentionally in zuul projects;
 # please don't submit patches that solely correct them or enable them.
-ignore = E124,E125,E129,E402,H,W503
+ignore = E124,E125,E129,E402,E741,H,W503,W504
 show-source = True
 exclude = .venv,.tox,dist,doc,build,*.egg