Remove instance Foreign Key in volumes table, replace with instance_uuid
Remove instance Foreign Key in volumes table, replace with instance_uuid

* Remove the instance relationship and instance_id FK
* Add instance_uuuid column to volumes table
* Passed unit tests and devstack tests

Change-Id: Id598f1f1d7915d1af6bf3dd75e5819dce08aaa0f

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