Use OS_CLOUD in sample local.sh

local.sh, if present, will be executed at the end of stack.sh. The
sample file here is meant to be copied to devstack root if desired.

Unfortunately, due to Change I86ffa9cd52454f1c1c72d29b3a0e0caa3e44b829
changing to use OS_CLOUD in stack.sh, sourcing openrc here will cause
both OS_CLOUD and traditional OS_* env vars to be set, which causes a
conflict.

Change-Id: Id80b46acab7d600ad7394ab5bc1984304825a672

samples/local.sh

@@ -31,7 +31,7 @@ if is_service_enabled nova; then
     # ``demo``)
 
     # Get OpenStack user auth
-    source $TOP_DIR/openrc
+    export OS_CLOUD=devstack
 
     # Add first keypair found in localhost:$HOME/.ssh
     for i in $HOME/.ssh/id_rsa.pub $HOME/.ssh/id_dsa.pub; do