Added check to see whether oslotest is installed

The projects that are using oslo.db in tests need to explicitly
include oslotest in their test-requirements.

Closes-bug: 1356425

Change-Id: I0ed01e141221d19af5593857c13a656f32d0f3cf

oslo/db/sqlalchemy/test_base.py

@@ -18,7 +18,13 @@ import functools
 import os
 
 import fixtures
-from oslotest import base as test_base
+
+try:
+    from oslotest import base as test_base
+except ImportError:
+    raise NameError('Oslotest is not installed. Please add oslotest in your'
+                    ' test-requirements')
+
 import six
 
 from oslo.db.sqlalchemy import provision