Encode metadata value to avoid double quoting

Using vars_prompt with the rax module creates metadata with extraneous u'' markup. Using self.encode() instead of repr() fixed the issue for me.


*Example playbook:*
```
#!/usr/bin/env ansible-playbook 
---
- name: Build Cloud Server
  hosts: localhost
  gather_facts: false
  vars_prompt:
    p_groups: "groups?"

  tasks:
    - name: "Server build request"
      local_action:
        module: "rax"
        credentials: "~/.rackspace_cloud_credentials"
        region: "DFW"
        name: "rstrox-{{ p_name }}"
        flavor: performance1-2
        image: "Ubuntu 14.04 LTS (Trusty Tahr) (PVHVM)"
        key_name: rstrox-key
        wait: yes
        state: present
        meta:
          groups: "{{ p_groups }}"
      register: rax
```

*Example result, rstrox-metadata's metadata was set by rax module.*

```
 % supernova $ACCOUNT list --fields 'name,metadata'
+--------------------------------------+--------------------+-------------------------------+
| ID                                   | Name               | Metadata                      |
+--------------------------------------+--------------------+-------------------------------+
| 719517b1-3183-4a86-b8de-xxxxxxxxxxxx | rstrox-esearch-1   | {u'groups': u'elasticsearch'} |
| 0859097e-e4f7-48ae-a322-xxxxxxxxxxxx | rstrox-esearch-2   | {u'groups': u'elasticsearch'} |
| d50a34f7-642d-4402-a8e0-xxxxxxxxxxxx | rstrox-metadata    | {u'groups': u"u'testvalue'"}  |
+--------------------------------------+--------------------+-------------------------------+

```