make the vm overcommit optional

Using this role on a virtual machine here, did not work, because that setting was not writeable by the root user. This was disabled by our host provider (http://serverfault.com/questions/82728/overcommitting-memory-on-an-ubuntu-server-9-10-running-on-vmware-fusion-3-0-osx/97321#97321)

For future references and Googlers:

```
failed: [91.250.114.88] => {"failed": true, "item": ""}
msg: checks_before failed with: key_path is not a writable file, key seems to be read only
```