The following enhancements are added:
-- PD keeps track of status of neutron routers: active or
standalone (master), or standby (not master),
-- PD DHCP clients are only spawned in the active router. In the
standby router, PD keeps track of the assigned prefixes, but
doesn't spawn DHCP clients.
-- When switchover occurs, on the router becoming standby, PD
clients are "killed" so that they don't send prefix withdrawals
to the DHCP server. On the router becoming active, PD spawns DHCP
clients with the assigned prefixes configured as hints in the
DHCP client's configuration
Closes-Bug: #1651465
Change-Id: I17df98128c7a88e72e31251687f30f569df6b860