{Reviewer - felix} Move HSS conf into /etc/clearwater/config

This change (and a corresponding Chef change) allow HSS configuration to be set in /etc/clearwater/config, fixing https://github.com/Metaswitch/crest/issues/15. I'm about to test it by:

* Spinning up a deployment
* Checking that the chef-configured values are passed on
* Logging onto the homestead node
* Editing /etc/clearwater/config
* Reinstalling homestead and checking that the new value is used
* Reinstalling homestead again and checking that the new value is not overwritten

I've also fixed https://github.com/Metaswitch/crest/issues/6 in the meantime. I still need to make the wiki updates to indicate the correct knife.rb and <environment>.rb values.