Clean up tenants created in test_users.py

Test_list_user_projects in test_users.py creates a tenant but
the tenant is not deleted at the end of the test.
This patch makes the tenant be deleted by adding addCleanup.

Fixes bug: 1179739

Change-Id: Ifc361363f74dcc43286e5fbdb582adf7f452dacc
1 file changed