Add endpoint for cloud subnets

[NickLaMuro]

Updated version of #1019

(just fixes some minor issues in the tests in the original, caused after review changes)

Addresses #1013

@miq-bot add_label enhancement, kasparov/no, laskar/no

Sample Requests

Create

Request

POST /api/cloud_subnets

{
  "action": "create",
  "resource": {
    "ems_id": 10000000000013,
    "name":   "test_cloud_subnet"
  }
}

Response

{
  "results": [
    {
      "success":   true,
      "message":   "Creating Cloud Subnet test_cloud_subnet for Provider: OpenStack Network Manager",
      "task_id":   "10000000053334",
      "task_href": "http://localhost:3000/api/tasks/10000000053334"
    }
  ]
}

Edit

Request

POST /api/cloud_subnets/10000000000028

{
  "action": "edit",
  "status": "available"
}

Response

{
  "success":   true,
  "message":   "Updating Cloud Subnet id: 10000000000028 name: 'provider'",
  "task_id":   "10000000053335",
  "task_href": "http://localhost:3000/api/tasks/10000000053335"
}

[NickLaMuro]

@miq-bot add_reviewer @agrare
@miq-bot add_reviewer @bdunne

As the two of you were originally on the reviewer list, figured you could also review the (slightly) revised version as well. Thanks.

[Fryguy]

Backported to morphy in commit 087772e.

commit 087772e85a497b2bbd9d401273b5addde09e29ce
Author: Adam Grare <agrare@redhat.com>
Date:   Fri Apr 9 10:03:40 2021 -0400

    Merge pull request #1025 from NickLaMuro/endpoints_for_cloud_subnets
    
    Add endpoint for cloud subnets
    
    (cherry picked from commit f6d2ec38716fafe3f5017b9187ba8826cfbe661c)