Description of problem:
When enabling IPv6 Prefix delegation, Neutron L3 agent is unable to start dibbler-client to fetch an IPv6 prefix from an upstream router.
There seems to be a rootwrap filter file missing (dibbler.filters), a failure when l3-agents want to launch a dibbler instance.


Version-Release number of selected component (if applicable):
openstack-neutron-common-8.1.2-1.el7.noarch
openstack-neutron-8.1.2-1.el7.noarch


How reproducible:
Always.


Steps to Reproduce:
1. /etc/neutron/neutron.conf: ipv6_pd_enabled = true
2. create a default IPv6 pool
   $ neutron subnetpool-create --shared --pool-prefix 2a01:xxxx:yyyy::/48 \
           --default-prefixlen 64 --is-default true prefix_delegation
3. create a tenant IPv6 network
   $ neutron net-create ipv6-pd
4. create a tenant IPv6 subnet using the above default pool
   $ neutron subnet-create ipv6-pd --name ipv6-pd-1 --ip_version 6 --ipv6_ra_mode slaac --ipv6_address_mode slaac --use_default_subnetpool
5. create a tenant router
6. attach tenant router to tenant network


Actual results:
No dibbler client instance is launched on network node.


Expected results:
A dibbler client instance is launched within the correct netns namespace.


Additional info:
Manually fetching the missing dibbler.filter file from openstack-neutron GIT repo <https://github.com/openstack/neutron/blob/master/etc/neutron/rootwrap.d/dibbler.filters> and placing it into /etc/neutron/rootwrap.d/ does solve the issue.