Clean up .gitignore

This cleans up .gitignore, removing things that no longer seem to be
relevant (e.g. .venv) and adding comments and formatting similar to
the suggestions proposed for openstack-dev/cookiecutter [1].

[1] https://review.openstack.org/#/c/592520/

Change-Id: I49ef78256f1f6f3942663441e7708536687b2363

.gitignore

@@ -1,13 +1,29 @@
-/build/
+# Add patterns in here to exclude files created by tools integrated with this
+# repository, such as test frameworks from the project's recommended workflow,
+# rendered documentation and package builds.
+#
+# Don't add patterns to exclude files created by preferred personal tools
+# (editors, IDEs, your operating system itself even). These should instead be
+# maintained outside the repository, for example in a ~/.gitignore file added
+# with:
+#
+# git config --global core.excludesfile '~/.gitignore'
+
+# Bytecompiled Python
+*.py[cod]
+
+# Packages
+*.egg-info
+
+# Unit test / coverage reports
+.coverage
+cover/
+.stestr/
 .tox/
-networking_powervm.egg-info/
-*.pyc
-.*.swp
-/.idea/
-doc/build
+
+# Sphinx
+doc/build/
+
+# pbr generates these
 AUTHORS
 ChangeLog
-.venv
-.coverage
-/.stestr/
-/cover/