[Reviewer: Alex] Remove gemini_enabled and memento_enabled options - always enable them

Alex,

Please can you review this change to fix https://github.com/Metaswitch/memento/issues/21 and https://github.com/Metaswitch/gemini/issues/11?

I've run the UTs and live-tested by spinning up a deployment without *_enabled set and confirming that gemini and memento services are still available.  There are corresponding tweaks to gemini and memento repos to correct docs, and a corresponding tweak to chef to remove these from /etc/clearwater/config - I won't bother you with those reviews.

Thanks,

Matt