Add autopep8 to tox and pre-commit

autopep8 is a code formating tool that makes python code pep8
compliant without changing everything. Unlike black it will
not radically change all code and the primary change to the
existing codebase is adding a new line after class level doc strings.

This change adds a new tox autopep8 env to manually run it on your
code before you submit a patch, it also adds autopep8 to pre-commit
so if you use pre-commit it will do it for you automatically.

This change runs autopep8 in diff mode with --exit-code in the pep8
tox env so it will fail if autopep8 would modify your code if run
in in-place mode. This allows use to gate on autopep8 not modifying
patches that are submited. This will ensure authorship of patches is
maintianed.

The intent of this change is to save the large amount of time we spend
on ensuring style guidlines are followed automatically to make it
simpler for both new and old contibutors to work on nova and save
time and effort for all involved.

Change-Id: Idd618d634cc70ae8d58fab32f322e75bfabefb9d

tox.ini

@@ -47,14 +47,29 @@ commands =
 description =
   Run style checks.
 envdir = {toxworkdir}/shared
+deps =
+  {[testenv]deps}
+  autopep8
 commands =
   {[testenv:mypy]commands}
+  # check if autopep8 would alter the formatting but don't actually change it
+  # so we can gate on this in the ci
+  autopep8 --exit-code --max-line-length=79 --diff -r nova doc setup.py
+  # since autopep8 only tries to make minimal changes to conform to pep8 we still need to run
+  # our hacking and flake8 check to keep our existing code style consistent.
+  # The full list of issues addressable by autopep8 can be found here
+  # https://pypi.org/project/autopep8/#features
   bash tools/flake8wrap.sh {posargs}
   # Check that all JSON files don't have \r\n in line.
   bash -c "! find doc/ -type f -name *.json | xargs grep -U -n $'\r'"
   # Check that all included JSON files are valid JSON
   bash -c '! find doc/ -type f -name *.json | xargs -t -n1 python -m json.tool 2>&1 > /dev/null | grep -B1 -v ^python'
 
+[testenv:autopep8]
+deps = autopep8
+commands =
+  autopep8 --exit-code --max-line-length=79 --in-place -r nova doc setup.py
+
 [testenv:fast8]
 description =
   Run style checks on the changes made since HEAD~. For a full run including docs, use 'pep8'