Add support for choices in steps
Add support for choices in steps

This adds ability to define choices in your step metadata and they will be
presented as a radio selection list.

Metdata used for this test:

```
title: Choose Kubernetes Network Plugin
description: Choose the type of network plugin you wish to use for your cluster
viewable: True
additional-input:
  - label: Network Plugin
    key: NETWORKPLUGIN
    type: choice
    default: flannel
    choices:
      - flannel
      - calico
```

Fixes #1167

Signed-off-by: Adam Stokes <49c255c1d074742f60d19fdba5e2aa5a34add567@users.noreply.github.com>

ubuntu/conjure-up,Ubuntu-Solutions-Engineering/conjure,conjure-up/conjure-up,conjure-up/conjure-up,ubuntu/conjure-up,Ubuntu-Solutions-Engineering/conjure