As part of Id56b9b70fb9c4113f4641e5b3f35ab63cfff6d4e, the special
handling of NEUTRON_DIR was removed. Readd it since it's needed to
install the proper version of neutron when testing neutron changes with
neutron-fwaas's functional tests. Otherwise those tests install neutron
HEAD.
Add a big comment to explain the reason for this.
Change-Id: Ibe1a057958f7c5b791788e2b44a1f13993a620bf