Devstack support for Neutron VPNaaS

Includes two enhancements to support VPNaaS and allow other
vendor implementation support. Goal is to provide a solution
for VPN .ini files until a more general solution can be defined
to handle all reference and vendor .ini files.

First, the /opt/stack/neutron/etc/vpn_agent.ini file is copied
to /etc/neutron, to allow the selected device drivers for agent
to be specified. Both reference and vendor implementations can
be uncommented in this file, prior to stacking.

Second, to allow vendor VPN implementations to provide an .ini
file to the agent at start-up time, an environment variable is
defined and can be set in the localrc to specify one or more
.ini files. The desire is NOT to include vendor settings in the
global vpn_agent.ini.

Change-Id: I5370ee57bbe326eff505a53055da4fddc0838de4
Closes-Bug: 1301552
Merge "Devstack support for Neutron VPNaaS"