Commit I9642ed9b513a43c5558f9611f43227299707284a rehomed the
PROVISIONAL_IPV6_PD_PREFIX constant into neutron-lib. This patch
consumes it removing the constant in neutron and using lib's version
of it instead.
NeutronLibImpact
Change-Id: I107cb5e0ff2f3e2c5bb9dc501f420d0be08735a0