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
Inject the domain name for devstack in tempest

A new setting for the keystone domain name is introduced in
tempest by change https://review.openstack.org/#/c/81872/.

The default value in tempest is set to None, so that it is
not devstack specific. It's up to devstack to configure
the name of the keystone domain defined in devstack.

Partially implements bp multi-keystone-api-version-tests

Change-Id: I3d1406407759e1fdce0704fa2d6aa025f23a5f08
Merge "Inject the domain name for devstack in tempest"
Merge "Devstack support for Neutron VPNaaS"