Minor improvement: to reduce layers in docker.

As each “ENV” and “RUN” lines create a new layer (I tested so, maybe
it’s just a mistake…), all the commands was aggregated to a single one.
Also, changed from CMD to ENTRYPOINT to call supervisord directly,
instead a shell script.
For future (Juno): keep the VM user untouched.

As seen in OpenStack documentation, when Heat is used to create VMs, a
new admin user (ec2-user) is created. For now it’s useless, but in Juno
release, the VM original user (ubuntu, debian, and so on) will be used,
despite of Heat scripts.
VPNaaS (issue #5) up and running.

I didn’t know (it’s good to read the docs :), but Neutron VPN agent
inherits L3 agent, so the first replaces the last one. The
openstack-neutron-agent::network Puppet class these resources (vpn and
l3 agents) was serialized to keep idempotency feature.