Network: enable instance deletion when dhcp release fails
Network: enable instance deletion when dhcp release fails

In the event that the 'dhcp_release' fails we should continue
with the deletion of the instance.

This will no longer the leave the instance in ERROR state.

Change-Id: Ie8259180a9df12865940907c728a8d9da3c9fda0
Closes-bug: #1289397

openstack/oslo.versionedobjects,citrix-openstack-build/oslo.versionedobjects