Fixes config_dir when installing haproxy from source

The current implementation will prepend the source prefix, which is fine, but then it just assumes your config dir was "/etc" which I believe is incorrect behavior.

This change respects the original `config_dir` value and just adds the prefix, so that **/etc/haproxy** will become **/usr/local/etc/haproxy**.

Lastly, since recipes have high attribute precedence than attributes, I have changed `node.set` to `node.default`. This produces the same behavior as before and allows a wrapper cookbooks to use `set` level instead of `override`.