Convert AWS::EC2::Subnet to use attributes Schema
Convert AWS::EC2::Subnet to use attributes Schema

Change-Id: I0f7b6beef1023875ae499ce7ec06f4211c6c028b

takeshineshiro/heat,steveb/heat,redhat-openstack/heat,dims/heat,rdo-management/heat,rh-s/heat,pshchelo/heat,pratikmallya/heat,dragorosson/heat,steveb/heat,dragorosson/heat,gonzolino/heat,jasondunsmore/heat,miguelgrinberg/heat,openstack/heat,cryptickp/heat,takeshineshiro/heat,pratikmallya/heat,rh-s/heat,noironetworks/heat,dims/heat,pshchelo/heat,maestro-hybrid-cloud/heat,jasondunsmore/heat,redhat-openstack/heat,cwolferh/heat-scratch,cwolferh/heat-scratch,miguelgrinberg/heat,srznew/heat,gonzolino/heat,noironetworks/heat,cryptickp/heat,maestro-hybrid-cloud/heat,srznew/heat,rdo-management/heat,openstack/heat