Refactor zebra model and type system

[rchamarthy]

Zebra model

The Zebra data model is defined by custom zebra resources that are extensible. We can add new resource types and register with the zebra server to manage the life cycle of these resources. The zebra API is a generic resource API used to create/update/delete, and read any resources. As zebra is geared toward a generic resource management system, it becomes essential to define a specific contract on how to define a new resource and plug it into the zebra server, client, and simulator services.

Resource Type contract

• Define a resource package under model with a package description. All resources in the package belong to the specific namespace.

model
  - types.go - registry of all types
  - compute
    - doc.go - contains compute package documentation
    - server.go - containers server resource definitions
    - ...
  - network
    - doc.go - contains network package documentation
    - switch.go - containers server resource definitions 
    - ...

• Define the resource type data structure with a unique type name, description and a constructor to create a new instance of the resource. Resource type name must be <namespace>.<type> format ensure that all names will be unique. Example type would be:

func ServerType() zebra.Type {
    return zebra.Type{
        Name:        "compute.server",
        Description: "a rack mountable compute server",
        Constructor: func() zebra.Resource { return new(Server) },
    }
}

• Resource definition must implement zebra.Resource interface
• Define a New<resourcename> method that produces a valid resource object

package compute

type Server struct {
...
}

func NewServer(name string, serial string) zebra.Resource {
....
}

• Define a function that would return a number of mock valid resource instances that can used for testing and simulation

func NewMockServerList(court int) zebra.ResourceList {
...
}

• Define a openAPI specification schema object for the resource. This will be used to refer in the zebra resource API specification.

{
  "type": "object",
  "required": ["name", "serialNumber"],
  "properties": {
    "name" : {
      "type": "string",
      "description": "host name of the server management board"
    },
    "serialNumber" : {
      "type": "string",
      "description": "serial number of the server"
    }
  }
}