Add .stestr to .gitignore
Add .stestr to .gitignore

With the move to stestr from testrepository, the name
of the folder is changed.

TrivialFix

Change-Id: Iea6be8f161f0c4dd2c27cc6729ccf6d25669447a

openstack/dragonflow,openstack/dragonflow,openstack/dragonflow