Add quantum_available config option

This lets us globally enable or disable tests that require quantum.

Change-Id: Id0452e98c6b21dcae92b79b545728e8444c474a3
diff --git a/etc/tempest.conf.sample b/etc/tempest.conf.sample
index a70a7ab..02bfdcb 100644
--- a/etc/tempest.conf.sample
+++ b/etc/tempest.conf.sample
@@ -193,6 +193,9 @@
 # for each tenant to have their own router.
 public_router_id = {$PUBLIC_ROUTER_ID}
 
+# Whether or not quantum is expected to be available
+quantum_available = false
+
 [volume]
 # This section contains the configuration options used when executing tests
 # against the OpenStack Block Storage API service