fix UnicodeEncodeError when prompt is unicode.

UnicodeEncodeError is occured when prompt text is unicode.

```
  File "/home/blah/Works/VEnvs/ansible/lib/python2.7/site-packages/ansible/callbacks.py", line 667, in on_vars_prompt
    result = prompt(msg, private)
  File "/home/blah/Works/VEnvs/ansible/lib/python2.7/site-packages/ansible/callbacks.py", line 655, in prompt
    return getpass.getpass(prompt)
  File "/usr/local/lib/python2.7/getpass.py", line 71, in unix_getpass
    passwd = _raw_input(prompt, stream, input=input)
  File "/usr/local/lib/python2.7/getpass.py", line 128, in _raw_input
    prompt = str(prompt)
```


```
---
- hosts: local
  gather_facts: no
  vars_prompt:
    - name: hello
      prompt: "Ĥȅľľő Ŵőŕľď!"
  tasks:
  - debug: msg=hello
```

This PR fixes this using encode(sys.stdout.encoding).

related PR: #3042