Different class names for VPNaaS migrations

In migrations 52ff27f7567a_support_for_vpnaas.py and
338d7508968c_vpnaas_peer_address_.py different class names are set:
neutron.services.vpn.plugin.VPNDriverPlugin and ne
utron.services.vpn.plugin.VPNPlugin.

Closes-bug: #1285641

Change-Id: Ieda8d3a8a022179867010b6e47d09cf7f43f5da9
Merge "Different class names for VPNaaS migrations"