When starting the dhcp-agent after an upgrade, there could
be stale IPv6 addresses in the namespace that had been
configured via SLAAC. These need to be removed, and the
same address added back statically, in order for the
agent to start up correctly.
To avoid the race condition where an IPv6 RA could arrive
while we are making this change, we must move the call
to disable RAs in the namespace from plug(), since devices
may already exist that are receiving packets.
Uncovered by the grenade tests.
Change-Id: I7e1e5d6c1fa938918aac3fb63888d20ff4088ba7
Closes-bug: #1627902
21bb776670