[Templates] Use str_replace for hosts.

There is a limit to how long input data can be for the heat script hook.

Change-Id: I48286e300874d7d8311d2ef14f95b6fc87ed579c
Closes-Bug: #1772673

hosts-config.yaml

@@ -11,15 +11,16 @@ resources:
     type: OS::Heat::SoftwareConfig
     properties:
       group: script
-      inputs:
-        - name: hosts
-          default:
-            list_join:
-              - ' '
-              - str_split:
-                - '\n'
-                - {get_param: hosts}
-      config: {get_file: scripts/hosts-config.sh}
+      config:
+        str_replace:
+          params:
+            WRITE_HOSTS:
+              list_join:
+                - ' '
+                - str_split:
+                  - '\n'
+                  - {get_param: hosts}
+          template: {get_file: scripts/hosts-config.sh}
 
 outputs:
   config_id:

scripts/hosts-config.sh

@@ -36,6 +36,7 @@ write_entries() {
 
 }
 
+hosts="WRITE_HOSTS"
 if [ ! -z "$hosts" ]; then
     for tmpl in /etc/cloud/templates/hosts.*.tmpl ; do
         write_entries "$tmpl" "$hosts"