Delete device when instance_id is null
Delete device when instance_id is null

When nfv delete action is performed, it helps to
remove the device from tacker when that device's
instance_id is null.

Change-Id: I3796a1c5b4a077d14c56ff84976e72a5a891fdca
Closes-bug: #1588713

openstack/tacker,openstack/tacker,openstack/tacker,stackforge/tacker,zeinsteinz/tacker,stackforge/tacker,priya-pp/Tacker,trozet/tacker,trozet/tacker,zeinsteinz/tacker,priya-pp/Tacker