Add vpnaas conf to Q_PLUGIN_EXTRA_CONF_FILES
Add vpnaas conf to Q_PLUGIN_EXTRA_CONF_FILES

A recent change[1] that adds lbaas conf files to
Q_PLUGIN_EXTRA_CONF_FILES has broken vpnaas devstack
plugin.

[1] https://review.openstack.org/#/c/366690

Change-Id: Ice58bce3c2bd469a52299b9c830b9b588d700acf
Closes-Bug: #1624976

openstack/neutron-vpnaas,openstack/neutron-vpnaas