If enable vpnaas,service_plugins value is 'neutron_vpnaas.services
.vpn.plugin.VPNDriverPlugin'.
It throw warning for 'stevedore.named [-]Could not load
neutron_vpnaas.services.vpn.plugin.VPNDriverPlugin'
Closes-Bug: #1690693
Change-Id: Ia00f733da2dcbdd50e3d62dfe98c8f44cc4a1d26