Enable version pinning from pillar version spec

Author: johnkeates

node/pkg.sls

@@ -21,3 +21,6 @@ nodejs:
   pkg.installed:
     - name: {{ node.node_pkg }}
     - reload_modules: true
+{%- if salt['pillar.get']('node:version') %}
+    - version: {{ node.version }}
+{%- endif %}