Fix for hostkey directory

I am using the new `accept_hostkey=yes` on the `git` module and I am getting the following error: `msg: /root/.ssh/ does not exist`. I tracked this down to the `add_host_key` method. 

The environment I am running this in looks like this:
```
        "SUDO_USER": "root",
        "USER": "vagrant",
        "USERNAME": "vagrant"
```

So it should not be looking into the `/root/.ssh` because I do not use `sudo` on the playbook or the play. I checked the `not_in_host_file` method and it looks within the `${USER}` known_hosts but when adding it, it does not. 

This solution will first check the USER environment variable or fallback to the old way, similar to the other methods. I have not been able to fully test this yet. Also, this could use some regression tests.