Make sure we remove old racks from resource_class during update
Make sure we remove old racks from resource_class during update

fixes bug 1212238

https://bugs.launchpad.net/tuskar/+bug/1212238

Change-Id: I4bb51282f1c5cbd9fa0ab5f77825fa95050e3298

rdo-management/tuskar,rdo-management/tuskar,rdo-management/tuskar