t3c log messages, variable names and flags assume only ATS is used #7723

This Improvement request (usability, performance, tech debt, etc.) affects these Traffic Control components:

• Traffic Control Cache Config (t3c, formerly ORT)

Current behavior:

t3c variables like tsConfigDir, flags like trafficserver-home and log messages are based on ATS being the cache server used. However, this is not the case any more with Varnish support.

New behavior:

A more generic names like cacheConfigDir or cache-home with logs using trafficserver or varnish based on what cache server type is using t3c would be more suitable.