Include the regions if they're supported with OPTIONS /api/providers

[skateman]

Requested by @Hyperkid123 in the issue below as a followup on #579, he needs the list of supported regions for providers that support it.

$ curl -u admin:smartvm -XOPTIONS "http://localhost:3000/api/providers"
    // ...
    "supported_providers": [
      // ...
      {
        "title": "Amazon EC2",
        "type": "ManageIQ::Providers::Amazon::CloudManager",
        "kind": "cloud",
        "regions": [
          {
            "name": "ap-south-1",
            "description": "Asia Pacific (Mumbai)"
          },
      // ...

Fixes #580
@miq-bot add_label changelog/yes, hammer/no, enhancement
@miq-bot assign @abellotti

I am a little worried about writing a spec for this, because I'm not 100% sure that we'd have a cloud provider neither that a cloud provider always has regions supported.

[Hyperkid123]

@skateman brilliant. Thank you!

[miq-bot]

Checked commit skateman@01ca75e with ruby 2.3.3, rubocop 0.52.1, haml-lint 0.20.0, and yamllint 1.10.0
1 file checked, 0 offenses detected
Everything looks fine. 🍪

[abellotti]

Thanks @skateman for the API enhancement. LGTM! 👍