Properly Catch RefsExistError on ressource creation

This patch is the (almost) cherry pick of
https://github.com/Juniper/contrail-neutron-plugin/commit/beaff643f64c3c6904b0ed09e57aef76ea8aa89

RefsExistError is not added for few of the resources
since it is already handled in neutron_plugin_db.py

Change-Id: I2dd80e9da1abc74a786cf32ea3c456606bcf5b97
Merge "Properly Catch RefsExistError on ressource creation"