Bug #910494: Horizon still tries to load quantum.client when QUANTUM_…

…ENABLED=False

Conditionally load api.quantum based on QUANTUM_ENABLED.  This means that
you don't have to have quantum.client installed if you're going to run with
QUANTUM_ENABLED=False.

Change-Id: I9abc08e18188a8fed681603ca2c0dd4534bc5135

horizon/horizon/api/__init__.py

@@ -36,4 +36,5 @@
 from horizon.api.keystone import *
 from horizon.api.nova import *
 from horizon.api.swift import *
-from horizon.api.quantum import *
+if settings.QUANTUM_ENABLED:
+    from horizon.api.quantum import *