FIXED_RANGE_V6 not set to sane value in neutron_plugins/services/l3

[Jay Faulkner]

http://logs.openstack.org/16/334816/21/check/gate-tempest-dsvm-ironic-ipa-wholedisk-agent_ipmitool-coreos-src-ubuntu-xenial/95f5e9e/logs/devstacklog.txt.gz#_2016-11-18_19_22_51_984

The awk statement here: https://github.com/openstack-dev/devstack/blob/master/lib/neutron_plugins/services/l3#L76 is only returning "64", leading to an invalid create command.

2016-11-18 19:40:58.306 | +++ lib/neutron_plugins/services/l3:_neutron_create_private_subnet_v6:306 : openstack --os-cloud devstack-admin --os-region RegionOne subnet create --project ded28149997744f2b64a840e53acb288 --ip-version 6 --subnet-range 64 --ipv6-ra-mode slaac --ipv6-address-mode slaac --network ac990965-9a51-400a-a41d-043362176698 ipv6-private-subnet

This is a bad request, as 64 is not a valid IPv6 subnet. Fix will be to fix the awk statement linked.