Fix ordering issue with private networks

[hyvs]

Close #253

This fix propose to switch regions_attributes to Set to follow API behavior.

Underlying OVH API returns unordered networks and trigger terraform changes during plan/apply :

First call

HTTP/1.1 200 OK
{
  "id": "pn-xxxxxx_43",
  "name": "private-network",
  "vlanId": 43,
  "regions": [
    {
      "region": "GRA7",
      "status": "ACTIVE",
      "openstackId": "********-****-****-****-************"
    },
    {
      "region": "SBG5",
      "status": "ACTIVE",
      "openstackId": "********-****-****-****-************"
    },
    {
      "region": "GRA5",
      "status": "ACTIVE",
      "openstackId": "********-****-****-****-************"
    },
    {
      "region": "GRA9",
      "status": "ACTIVE",
      "openstackId": "********-****-****-****-************"
    }
  ],
  "type": "private",
  "status": "ACTIVE"
}

Second call

HTTP/1.1 200 OK
{
  "id": "pn-xxxxxx_43",
  "name": "private-network",
  "vlanId": 43,
  "regions": [
    {
      "region": "GRA5",
      "status": "ACTIVE",
      "openstackId": "********-****-****-****-************"
    },
    {
      "region": "SBG5",
      "status": "ACTIVE",
      "openstackId": "********-****-****-****-************"
    },
    {
      "region": "GRA7",
      "status": "ACTIVE",
      "openstackId": "********-****-****-****-************"
    },
    {
      "region": "GRA9",
      "status": "ACTIVE",
      "openstackId": "********-****-****-****-************"
    }
  ],
  "type": "private",
  "status": "ACTIVE"
}