Fix import in nova cli tests

The min version decorator was references the full qualified name for
the decorator but the cli module is imported as just cli in the test
module. Why this isn't causing an error remains unknown (it fails
flake8 locally on my system) but regardless this patch fixes the
issue and corrects the call.

Change-Id: I916d6621ab93900355fa1cb2b114afecbdc60b33
diff --git a/tempest/cli/simple_read_only/compute/test_nova.py b/tempest/cli/simple_read_only/compute/test_nova.py
index 4fe4982..5efeb75 100644
--- a/tempest/cli/simple_read_only/compute/test_nova.py
+++ b/tempest/cli/simple_read_only/compute/test_nova.py
@@ -149,7 +149,7 @@
     def test_admin_secgroup_list_rules(self):
         self.nova('secgroup-list-rules')
 
-    @tempest.cli.min_client_version(client='nova', version='2.18')
+    @cli.min_client_version(client='nova', version='2.18')
     def test_admin_server_group_list(self):
         self.nova('server-group-list')