Comment 9 for bug 1675778

Looks like a race between ipv6 subnet create and network dhcp port create.

Neutron server adds an ipv6 address to a dhcp port in two cases:

 1) network already has ipv6 subnet by the time dhcp agent requests dhcp port creation - in this case agent includes both subnets into requested IPs of the port and both get allocated;
 2) ipv6 subnet is created after the network already has dhcp port existing - ipv6 IP then gets allocated on the dhcp port as part of subnet creation on the server side;

The bug reveals the third case:
 3) ipv6 subnet and dhcp port are created at the same time: so no ipv6 IP is requested for dhcp port by dhcp agent, as well as no ipv6 address is added to dhcp port as part of subnet creation;

In this case dhcp agent tries to reprocess network after subnet/port creation and updates IPs on the dhcp port:

 2017-03-30 05:12:38.990 29848 DEBUG neutron.api.rpc.handlers.dhcp_rpc [req-bcd62396-0e9a-4f39-8bf7-e56f0588805c - - - - -] Update dhcp port {u'port': {u'network_id': u'29d6752b-027a-4eb9-aa73-711eff1b58ca', 'binding:host_id': u'node-2.test.domain.local', u'fixed_ips': [{u'subnet_id': u'3f81f975-5718-4bdc-878c-614f22b1b783', u'ip_address': u'192.168.100.2'}, {u'subnet_id': u'8363ac60-c30d-43dc-a1d1-3d39820602fd'}]}, 'id': u'2e3a5343-a995-498a-85d2-db686d119fab'}

Server ignores ipv6 auto-address subnets in this request - to be fixed. This should be handled as in case 3, see proposed patch below.