Add spec for retargetable functional testing

This spec proposes the addition of support for retargetable
functional testing.  Retargetable tests are intended to be written
to an abstract client interface that can have multiple backends,
allowing an API test to be written that can be run against the
plugin API and a Neutron service with minimal extra effort.

bp retargetable-functional-testing

Change-Id: I1aa1e147012f853d3c192de80505e6ce0d50b5c4
Merge "Add spec for retargetable functional testing"