Adding fields "post_node" and "collection_set" to Node class


1) add_fields_to_existing_documents.py
   - Code added in order to add/update following fields: post_node and collection_set

2) models.py
   - post_node: New field added to Node class
   - collection_set: This field is moved from GSystem class to Node class

* IMPORTANT - Please run following command to insert default values for above specified fields in existing documnets:
  - python manage.py add_fields_to_existing_documents

3) filldb.py
   - Bug resolved: MetaType[0] changed to META_TYPE[0]
 
4) ndf_tags.py
   - Updated get_gapps_menubar() function to display GAPPS in proper order.