Release as a plugin

As a Mirantis Engineer I want to express a Fuel Release as a Fuel Plugin so
that I could define, maintain and deploy various flavors of customized
OpenStack deployments in a clean isolated way, externalized from common
Fuel provisioning layer.

The nailgun repo sill holds onto one of the remaining parts of the data model
the release fixture. This fixture is used to describe everything about the
deployment from the ground up and is where every change can possibly be
expressed.

By moving the release fixtures ``openstack.yaml`` completely into the plugin
framework we opening road to following changes:

* To make ``fuel-library`` repo a plugin.
* It is possible to ship multiple openstack version release packages as
  each is its own plugin.
* Next steps allowing Fuel to have different releases bundled or no pre-bundled
  releases at all (lightweight version) are possible as well.