Fix undefined attribute in scenario test class

An attribute check is being performed: "if not self.image_ref"
but self.image_ref may not be defined at all, leading to the
following error. So, let's define it first so it can be properly
evaluated.

AttributeError: 'TestShareBasicOpsNFS' object
has no attribute 'image_ref'

TrivialFix

Change-Id: I0c454009b95c5cb0b6fc1304bb0cba1ff87c9fe9
diff --git a/manila_tempest_tests/tests/scenario/test_share_basic_ops.py b/manila_tempest_tests/tests/scenario/test_share_basic_ops.py
index 304d4e3..10218a5 100644
--- a/manila_tempest_tests/tests/scenario/test_share_basic_ops.py
+++ b/manila_tempest_tests/tests/scenario/test_share_basic_ops.py
@@ -51,6 +51,7 @@
         base.verify_test_has_appropriate_tags(self)
         # Setup image and flavor the test instance
         # Support both configured and injected values
+        self.image_ref = None
         if not hasattr(self, 'flavor_ref'):
             self.flavor_ref = CONF.share.client_vm_flavor_ref
         if CONF.share.image_with_share_tools: