Nova conf:don't use deprecated option "fixed_key" from group "keymgr"

That option now lives in the "key_manager" group.

Change-Id: I35c2914eaf2a190d6494107260483a6835de3e47
diff --git a/stack.sh b/stack.sh
index 3de9af2..1ca66b0 100755
--- a/stack.sh
+++ b/stack.sh
@@ -1208,9 +1208,9 @@
     done
 fi
 
-# Create a randomized default value for the keymgr's fixed_key
+# Create a randomized default value for the key manager's fixed_key
 if is_service_enabled nova; then
-    iniset $NOVA_CONF keymgr fixed_key $(generate_hex_string 32)
+    iniset $NOVA_CONF key_manager fixed_key $(generate_hex_string 32)
 fi
 
 # Launch the nova-api and wait for it to answer before continuing