Add find_entities helper method, for easy conversion between name and ID

This is the function that I'm having to define in each piece of code that uses flapjack diner 1.0.

@ali-graham What are your thoughts on including this into diner before 2.0 comes out?  
The reason I did it this way was to allow people to give short names (eg, db-app-01) and get back the entities, with ids, matching it.  Do we really want to force that a full FQDN has to be given for every search?

I like the idea of a regex here - it would make this function more useful.