In case when L3 agent is hosting routers which have got subnets
with Prefix Delegation enabled, agent couldn't properly handle
IpAddressAlreadyExists exception raised when pd module tries to
configure link local IPv6 addresses.
Now this is fixed and L3 agent can restart without problems in such
case.
Change-Id: Icc995f7b2b465921e41342711d17539f16ead0ce
Closes-Bug: #1892362