Update _TestModelMigration
Update _TestModelMigration

_TestModelMigrationTest contain some duplication of code from oslo.db.
As change 94d583acde16860e7ca535bd7a960e8993cd12d2 was merged and
included in oslo.db 2.4.1 this duplication can be avoid.

Change-Id: I5e789fb2fd6a27c1778b6155afc3d0396ccb5cb9

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