Engine: optional Logging field to disable custom logging in engine.

* The default behavior is preserved
* This disables the use of the `TEST` environment variable in engine.

Docker-DCO-1.1-Signed-off-by: Solomon Hykes <solomon@docker.com> (github: shykes)
Merge pull request #4166 from shykes/engine-logging

Engine: optional Logging field to disable custom logging in engine.