Fix oslotest import error

The barbican-tempest-plugin gate has stopped working
after export DEVSTACK_GATE_TEMPEST_ALL_PLUGINS=1 was
removed in Ibef3f9a135f14727bf57c29e766f838d7da56c68
since oslotest is no longer installed.

This patch uses the tempest.lib base test instead
of using the oslotest base test.

Change-Id: Ifdf8b426c21e4b3a51f97cbc3d95eb842eb04515
diff --git a/barbican_tempest_plugin/tests/base.py b/barbican_tempest_plugin/tests/base.py
index 1f97635..600ba80 100644
--- a/barbican_tempest_plugin/tests/base.py
+++ b/barbican_tempest_plugin/tests/base.py
@@ -12,7 +12,7 @@
 # License for the specific language governing permissions and limitations
 # under the License.
 
-from oslotest import base
+from tempest.lib import base
 
 
 class TestCase(base.BaseTestCase):