Fix lower-constraints.txt
Fix lower-constraints.txt

During the change https://review.openstack.org/#/c/573216/
neutron-vpnaas lower-constraints.txt looks out-of-date.
This commit fixes lower-constraints.txt.

Change-Id: Id9cfb463a98bdcc3c45505d8701c515549ecaa55

openstack/python-openstackclient,openstack/python-openstackclient,dtroyer/python-openstackclient,dtroyer/python-openstackclient