Makes vpn plugin configurable

Currently, it is hardcoded and not good for those who have their
own vpn plugin developed.

Change-Id: I6244040dd2abea016556cd515e06f53edbb993a0
Closes-bug: 1339977
diff --git a/lib/neutron_plugins/services/vpn b/lib/neutron_plugins/services/vpn
index d920ba6..2478c47 100644
--- a/lib/neutron_plugins/services/vpn
+++ b/lib/neutron_plugins/services/vpn
@@ -7,7 +7,7 @@
 
 
 AGENT_VPN_BINARY="$NEUTRON_BIN_DIR/neutron-vpn-agent"
-VPN_PLUGIN="neutron.services.vpn.plugin.VPNDriverPlugin"
+VPN_PLUGIN=${VPN_PLUGIN:-"neutron.services.vpn.plugin.VPNDriverPlugin"}
 IPSEC_PACKAGE=${IPSEC_PACKAGE:-"openswan"}
 
 function neutron_vpn_install_agent_packages {