Description of problem:
During update from 7.0 to 7.1 on HA deployment(3 x ctrls + 1 x compute) with network isolation the overcloud instances lose connectivity via their floating IPs.

Steps to Reproduce:
1. Deploy 7.0
openstack overcloud deploy --templates ~/templates-7.0/my-overcloud -e ~/templates-7.0/my-overcloud/environments/network-isolation.yaml -e ~/templates-7.0/network-environment.yaml  --control-scale 3 --compute-scale 1 --ntp-server clock.redhat.com --libvirt-type qemu

2. Create external network, tenant network, router on the overcloud network.

3. Boot instance and assign it a floating IP on the external network 

4. Update undercloud to 7.1

5. Update stack
openstack overcloud update stack overcloud -i --templates ~/templates-7.1/my-overcloud -e ~/templates-7.1/my-overcloud/overcloud-resource-registry-puppet.yaml -e ~/templates-7.1/my-overcloud/environments/network-isolation.yaml -e ~/templates-7.1/network-environment.yaml  -e ~/templates-7.1/update.yaml

6. Check connectivity to the instance floating IP

Actual results:
There's no connectivity during the update.

Expected results:
The instance keeps connectivity during the update.

Additional info:
I see the l3 agent ha_state in different states during update: active on 2 nodes, active on all of the nodes.

At the end the update fails and there's a single alive l3 agent but it's got a standby ha_state. 

stack@instack:~>>> neutron l3-agent-list-hosting-router tenant-router
+--------------------------------------+------------------------------------+----------------+-------+----------+
| id                                   | host                               | admin_state_up | alive | ha_state |
+--------------------------------------+------------------------------------+----------------+-------+----------+
| aef76e90-1157-46cf-a805-2059975639f8 | overcloud-controller-1.localdomain | True           | :-)   | active   |
| 2b195c24-65a1-4923-a506-5dba365d7d5c | overcloud-controller-0.localdomain | True           | :-)   | standby  |
| f8e2331e-fc16-4c7b-a8f3-4a51fd0e3406 | overcloud-controller-2.localdomain | True           | :-)   | active   |
+--------------------------------------+------------------------------------+----------------+-------+----------+
stack@instack:~>>> neutron l3-agent-list-hosting-router tenant-router
+--------------------------------------+------------------------------------+----------------+-------+----------+
| id                                   | host                               | admin_state_up | alive | ha_state |
+--------------------------------------+------------------------------------+----------------+-------+----------+
| aef76e90-1157-46cf-a805-2059975639f8 | overcloud-controller-1.localdomain | True           | :-)   | active   |
| 2b195c24-65a1-4923-a506-5dba365d7d5c | overcloud-controller-0.localdomain | True           | :-)   | active   |
| f8e2331e-fc16-4c7b-a8f3-4a51fd0e3406 | overcloud-controller-2.localdomain | True           | :-)   | active   |
+--------------------------------------+------------------------------------+----------------+-------+----------+
stack@instack:~>>> neutron l3-agent-list-hosting-router tenant-router
+--------------------------------------+------------------------------------+----------------+-------+----------+
| id                                   | host                               | admin_state_up | alive | ha_state |
+--------------------------------------+------------------------------------+----------------+-------+----------+
| aef76e90-1157-46cf-a805-2059975639f8 | overcloud-controller-1.localdomain | True           | :-)   | standby  |
| 2b195c24-65a1-4923-a506-5dba365d7d5c | overcloud-controller-0.localdomain | True           | xxx   | active   |
| f8e2331e-fc16-4c7b-a8f3-4a51fd0e3406 | overcloud-controller-2.localdomain | True           | xxx   | standby  |
+--------------------------------------+------------------------------------+----------------+-------+----------+