Allow setting os environment vars in python::gunicorn

Adding the ability to set os environment variables for the gunicorn process. This adds the "osenv" attribute to `python::gunicorn` which accepts a hash of `'key': 'value'` pairs. This defaults to `False` which keeps backwards compatibility in the vhost gunicorn config file.

For example you could set an os environment variable that allows you to set a gunicorn app's database server with:
```
python::gunicorn { 'vhost':
  osenv       => { 'DBHOST' => 'dbserver.example.com' },
}
```
Which would add the following to `/etc/gunicorn.d/vhost.conf`:
```
...
'environment': {
    'DBHOST': 'dbserver.example.com',
...
```

Resolves #132