Avoid to overrdie entire policy file

We should not override entire policy file, this might cause error to
other services (like Tacker) who also try to override Heat policy file.

Change-Id: I349de4b5e78fd6807db646543e3ac69212f304e9

devstack/plugin.sh

@@ -342,11 +342,11 @@ function stop_vitrage {
 
 function modify_heat_global_index_policy_rule {
     if is_service_enabled heat; then
-        cat << EOF > /etc/heat/policy.yaml
-# List stacks globally.
-# GET  /v1/{tenant_id}/stacks
-"stacks:global_index": "rule:deny_stack_user"
-EOF
+        local heat_policy=/etc/heat/policy.yaml
+        touch $heat_policy
+        # List stacks globally.
+        # GET  /v1/{tenant_id}/stacks
+        echo '"stacks:global_index": "rule:deny_stack_user"' >> $heat_policy
     fi
 }