If the formula was ran on a system that reported an os_family that
wasn't one of "Debian", "RedHat", "Arch", "Suse" then the map.jinja
template would fail to render with "'NoneType' is not iterable.

This occurs because grains.filter_by will return None when it fails
match the grain to the input dictionary. The value is then blindly
passed into a dict.update() which causes the failure.

In this patch we ensure that the default values, as defined in
defaults.yaml, are always applied when grain matching fails.