Set path for neutron conf dir in devstack if missing
Set path for neutron conf dir in devstack if missing

The variable NEUTRON_CONF_DIR is no longer available to the
script for functional setup. This defaults it to the correct
location if it's not already set.

Closes-Bug: #1579946
Change-Id: I060aea1de70d7939a8ea68771702f74a00d88fe9

openstack/neutron-vpnaas,openstack/neutron-vpnaas