delete_port: ensure quota usage is marked as dirty
delete_port: ensure quota usage is marked as dirty

To this aim the ORM session mapper must be used.
This patch simply uses context.session.delete rather than
query.delete, and handles UnmappedInstanceError to safely
complete the operation when the record is deleted by
another transaction.

Change-Id: I55c701fc1e2fda4461501aae532bbe11cce45b75
Closes-Bug: #1497459

sebrandon1/neutron,bigswitch/neutron,mahak/neutron,klmitch/neutron,apporc/neutron,swdream/neutron,dims/neutron,dhanunjaya/neutron,eayunstack/neutron,cloudbase/neutron,wolverineav/neutron,mahak/neutron,jumpojoy/neutron,huntxu/neutron,chitr/neutron,klmitch/neutron,apporc/neutron,bigswitch/neutron,eayunstack/neutron,noironetworks/neutron,asgard-lab/neutron,asgard-lab/neutron,dims/neutron,noironetworks/neutron,SamYaple/neutron,glove747/liberty-neutron,huntxu/neutron,igor-toga/local-snat,cloudbase/neutron,yanheven/neutron,SamYaple/neutron,chitr/neutron,swdream/neutron,wolverineav/neutron,sebrandon1/neutron,openstack/neutron,glove747/liberty-neutron,jumpojoy/neutron,openstack/neutron,yanheven/neutron,JianyuWang/neutron,JianyuWang/neutron,igor-toga/local-snat,MaximNevrov/neutron,dhanunjaya/neutron,MaximNevrov/neutron,mahak/neutron,openstack/neutron