Allow setting os environment vars in python::gunicorn

[derektamsen]

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