Add pylint with indent check

This is an initial creation of pylint with a basic indent checker.
Small issues corrected.  Job added to gate with
Ib554a284e92583cc1d6a5c2219b3922852ca4c73

Change-Id: I7e24d8348db3aef79e1395d12692199a1f80161a
Co-Authored-By: Andreas Florath <andreas@florath.net>

tox.ini

@@ -16,6 +16,11 @@ commands =
   flake8 {posargs}
   dib-lint
 
+[testenv:pylint]
+setenv = VIRTUAL_ENV={envdir}
+deps = pylint
+commands = pylint --rcfile pylint.cfg diskimage_builder
+
 [testenv:venv]
 commands = {posargs}