Add .stestr to gitignore
Add .stestr to gitignore

Trivial but useful.

Change-Id: I8717cfd43d260d9a86269057b8caa516046717cd

openstack/sahara-scenario,openstack/sahara-tests,openstack/sahara-tests,openstack/sahara-scenario,openstack/sahara-tests,openstack/sahara-tests,openstack/sahara-scenario