Fix vars_prompt no/false default values

__Issue Type:__ Bug report

__Ansible Version:__ 1.8.2

__Environment:__ N/A

__Summary:__ `no` and `false` default values in `vars_prompt` become `""`

__Steps to Reproduce:__ 
```
---
- name: Default values of prompts
  hosts: localhost
  gather_facts: no
  vars_prompt:
    - name: no_val_from_prompt
      prompt: Test default boolean value - press enter
      default: no
      private: no
  tasks:
    - debug: var=no_val_from_prompt
```

__Expected Results:__
```
ok: [localhost] => {
    "no_val_from_prompt": "False"
}
```

__Actual Results:__
```
ok: [localhost] => {
    "no_val_from_prompt": ""
}
```