Add hacking import exception for service clients

This commit adds import exceptions to the service client imports.
The service clients are imported at the class level and it would be
a major effort to change this. However, we want to be enforcing H302
everywhere else. This commit adds an exception to allow importing the
service clients as classes.

Change-Id: Ife74539fe04e97740e45325140f407fd7476e1c7
diff --git a/tox.ini b/tox.ini
index 5e8d283..ef49b28 100644
--- a/tox.ini
+++ b/tox.ini
@@ -95,6 +95,7 @@
 
 [hacking]
 local-check-factory = tempest.hacking.checks.factory
+import_exceptions = tempest.services
 
 [flake8]
 # E125 is a won't fix until https://github.com/jcrocholl/pep8/issues/126 is resolved.  For further detail see https://review.openstack.org/#/c/36788/