Refine the OpenStackSecurityGroup API

[pierreprinetti]

The CRD fields should reflect the OpenStack REST API as closely as possible, while conforming to the Kubernetes best practices.

Ideally, this task is resolved when:

• the spec only contain fields and options described in the OpenStack REST API reference for the resource creation;
• each field and property from the OpenStack API that is missing in the CRD (e.g. because it doesn't fit the MVP) is added commented out;
• the controller correctly handles the spec;
• the status contains all the fields described as a response to the OpenStack REST API "GET" operation on the resource;
• the controller correctly populates the status object upon reconcile.

Non-goals:

• having EVERY field of the REST API in the spec.