Avoid double colon with 'variable: "prompt"' style

There is a little minor flaw with vars_prompt.
```yaml
---
- hosts: somehosts
  user: root

  vars_prompt:
    var1: "Name of your bridge?"
```
There are two colons.
```bash
Name of your bridge?: : 
```
When I use the 'extended' variant for the prompt.

```yaml
---
- hosts: somehosts
  user: root

  vars_prompt:
    - name: "var1"
      prompt: "Name of your bridge?"
      default: "vnet0"
      private: no
```
The prompt looks like this:
```bash
Name of your bridge?:
```
This pull request should fix the issue. Beside running the two examples from above I tested it with '--extra-vars="var1=vnet0"' and it seems that the commit have no influence on the variable. It would be nice if somebody could check it and confirm that the double colon occurs.