Hostname field should validate input

[renatomefi]

I was trying some variations at phansible web interface and when I put "phansible_issue_182" as my hostname the ansible was not able to set this hostname for the guest (of course), I can see two possible solutions:
1- Validate the field;
2- Prepare the hostname before generating the file, removing unwanted chars.

Here is the error for those looking at ansible errors at google:

TASK: [vagrant_local | Set the hostname] ************************************** 
failed: [192.168.33.101] => {"changed": true, "cmd": "hostname phansible_issue_185_2", "delta": "0:00:00.002881", "end": "2015-05-07 19:50:04.758369", "rc": 1, "start": "2015-05-07 19:50:04.755488", "warnings": []}
stderr: hostname: the specified hostname is invalid

[InFog]

I think just removing unwanted chars is good enough. Do you want to prepare something like this, @renatomefidf? If so, please, do it. Otherwise I'll :)

[debo]

@renatomefidf I do prefer option one, I'll work on it.