When the external gateway is plugged and we enable IPv6
forwarding on it, make sure the 'all' sysctl knob is also
enabled, else IPv6 packets will not be forwarded. This
seems to only affect HA routers that default to disabling
this 'all' knob on creation.
Also, when we are removing all the IPv6 addresses from a
HA router internal interface, set 'accept_ra' to zero so
it doesn't accidentally auto-configure an address. Set
it back to one when adding them back.
Re-homed newly added _wait_until_ipv6_forwarding_has_state()
accordingly.
Conflicts:
neutron/tests/functional/agent/l3/test_ha_router.py
Closes-bug: #1787919
Change-Id: Ia1f311ee31d1479089685367a97bf13cf170b342
(cherry picked from commit b847cd02c5)
(cherry picked from commit dfedafe5f6)