The dvr extension's API definition was rehomed into neutron-lib with
commit I6d98e83287833882515240d7db1faa4a1deb7da1 and will be consumed
in neutron with Id5f0be33b036370b7545258a321830fdaa4dc0da
This patch consumes the API def by using neutron-lib's implementation
rather than neutrons.
Change-Id: I1d06c3d607fce1c3887ff121fee7e9132c1ea521