Clear out the keystone pki data for each run

This prevents old invalid tokens from working after a rerun of stack.sh
and potentially providing users and tenants that don't exist.

Fixes bug 1089700

Change-Id: Icfc22978e41e459d51b50bc7ad2e6d98d766e402
diff --git a/lib/quantum b/lib/quantum
index 288a327..480aaa1 100644
--- a/lib/quantum
+++ b/lib/quantum
@@ -127,6 +127,7 @@
     # Create cache dir
     sudo mkdir -p $QUANTUM_AUTH_CACHE_DIR
     sudo chown `whoami` $QUANTUM_AUTH_CACHE_DIR
+    rm -f $QUANTUM_AUTH_CACHE_DIR/*
 }
 
 function quantum_setup_ovs_bridge() {