Add __pycache__ to .gitignore

This commit adds __pycache__ to .gitignore. __pycache__ directories are
made by python 3 execution and we check un-ignored files after the tests
in the gate jobs. And a lot of __pycache__ directories are listed in the
logs. However, it is expected directories so it should be ignored.

Change-Id: I4cfd3475dd2fe1e5b586a28296983a3914236d70

.gitignore

@@ -1,6 +1,7 @@
 AUTHORS
 ChangeLog
 *.pyc
+__pycache__/
 etc/tempest.conf
 etc/tempest.conf.sample
 etc/logging.conf