The L3_AGENT_MODE_DVR_NO_EXTERNAL and DVR_SNAT_BOUND constants were
rehomed into neutron-lib with Ieb9374f5483a0ab2306592ab901686ca374db1c8
This patch consumes them by removing them from neutron and using the
constants from neutron-lib instead.
NeutronLibImpact
Change-Id: Ib63a523721a2fa3d1a978a729de28e6a2e560ef6