This patch rehomes the SUBPORTS, TRUNK and TRUNK_PLUGIN constants
from neutron.services.trunk.constants into the neutron-lib callback
resources as they are used in callbacks today [1].
[1] http://codesearch.openstack.org/?q=%5C.(SUBPORTS%7CTRUNK%7CTRUNK_PLUGIN)%2C
Change-Id: I26d9e748cdf1425d206d3e63056986011a518d6f