detect failure of 'keystone token-get'

When 'keystone token-get' fails, the caller can't detect the failure.
This cause troulbe shooting a bit complicated.

Change-Id: I3c58c5fd0e92a87e87546ea797904e08646a1097
Closes-Bug: #1238412
diff --git a/stack.sh b/stack.sh
index 7cd7e30..024c52e 100755
--- a/stack.sh
+++ b/stack.sh
@@ -1203,6 +1203,7 @@
 
 if is_service_enabled g-reg; then
     TOKEN=$(keystone token-get | grep ' id ' | get_field 2)
+    die_if_not_set $LINENO TOKEN "Keystone fail to get token"
 
     if is_baremetal; then
        echo_summary "Creating and uploading baremetal images"