devstack gate hook for fullstack job, fix IPV4_ADDRS_SAFE_TO_USE
devstack gate hook for fullstack job, fix IPV4_ADDRS_SAFE_TO_USE

Set IPV4_ADDRS_SAFE_TO_USE to the same value used by devstack-vm-gate.sh
to avoid issues in cloudcity.

This is temporary until we use devstack-vm-gate.sh
(see Ib2a4f1f258d39dcba370fe37799c56ad0e5ca8fd).

Change-Id: I77ad056a0a1ec0e4d0282ae3dbb002641635f043
Partial-Bug: 1693689

stackforge/networking-bagpipe-l2,openstack/networking-bagpipe,openstack/networking-bagpipe-l2,openstack/networking-bagpipe,stackforge/networking-bagpipe-l2,openstack/networking-bagpipe-l2