back_ref_fields is a list. so can't call add to set

Change-Id: Ia71ece1549a64d0be243f0b03ad05be4fd9599b6
Merge remote-tracking branch 'upstream/R1.06' into R1.06
Revert "Support subnet name using kv_store"

This reverts commit b8f6f17d81eb0d05254dc5eb5339886601bec743.

Conflicts:
	neutron_plugin_contrail/plugins/opencontrail/ctdb/config_db.py