exclusive_resources: use set remove instead of discard

If the release function cannot find a resource in the allocation
pool, it should raise an error.

Change-Id: Ieffd547f5c08fbe56289c99dc0384e7bd8a8f85b

neutron/tests/common/exclusive_resources/resource_allocator.py

@@ -88,7 +88,7 @@ class ResourceAllocator(object):
     @utils.synchronized('resource_allocator', external=True, lock_path='/tmp')
     def release(self, resource):
         allocations = self._get_allocations()
-        allocations.discard(resource)
+        allocations.remove(resource)
         if allocations:
             self._write_allocations(allocations)
         else:  # Clean up the file if we're releasing the last allocation