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: I492b20c8464aff6aa9deb53639e623dcfb2cf9bc

.gitignore

@@ -1,11 +1,29 @@
-/build/
-.stestr/
-.tox/
-ceilometer_powervm.egg-info/
-*.pyc
-doc/build
-AUTHORS
-ChangeLog
-/.idea/
+# 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/
+
+# Sphinx
+doc/build/
+
+# pbr generates these
+AUTHORS
+ChangeLog