Replace test.attr with decorators.attr
Replace test.attr with decorators.attr

Function 'tempest.test.attr()' has moved to 'tempest.lib.decorators
.attr()' in Pike and will be removed in a future version.
This patch replaces the 'tempest.test.attr()' with the 'tempest.lib
.decorators.attr().'

Change-Id: Ia1ed973398e801dfb5f46e172515dbf520ce4c43

openstack/neutron-vpnaas,openstack/neutron-vpnaas