Change usage of ostf repos to one

Replace usage of ostf_test_sha and ostf_plugin_sha
to ostf_sha only in next files:

* nailgun.api.handlers.version.py
* nailgun.settings.yaml
* nailgun.test.unit.test_version_handler
* nailgun.wsgi.py

Change-Id: Ieeff26e97cad7e39afe1ca400b1a81f553920a33