Added engine healthcheck to API, CLI and UI

A simple mechanism to check the health of engines in the Shipyard
cluster.  It functions by performing a HTTP GET against /v1.15/info on
each engine every 10 seconds.

It's pretty rudimentary at the moment.  I'll have a look at 
 - seeing if the HTTP GET requests can be dispatched concurrently
 - setting a default status for engines if they haven't had a health check performed