Set defaults in a separate (private) recipe

Since one can define broker configuration options in a number of
different ways we cannot really set broker defaults in an attributes
file.

Instead we set them in a `_defaults` recipe, but only if they aren’t
already set to something, as we don’t want to override what a user has
already set.

Closes #55.
Version bump to v0.5.1

Bug fix release with a fix for #55.