Extract our default PATH value to a constant for great reuse

Docker-DCO-1.1-Signed-off-by: Andrew Page <admwiggin@gmail.com> (github: tianon)
Engine: clearer job error reporting

* Job.Error and Job.Errorf always print a newline.
* Avoid redundant prefixing of error messages.

Docker-DCO-1.1-Signed-off-by: Solomon Hykes <solomon@docker.com> (github: shykes)
Engine: add tests for ParseJob()

Docker-DCO-1.1-Signed-off-by: Solomon Hykes <solomon@docker.com> (github: shykes)
Engine: tiny cosmetic fix

To make @creack happy :)

Docker-DCO-1.1-Signed-off-by: Solomon Hykes <solomon@docker.com> (github: shykes)
Merge pull request #4312 from shykes/engine-error-reporting

Engine: clearer job error reporting
Merge pull request #4290 from tianon/extract-path

Extract our default PATH value to a constant for great reuse
Merge pull request #4320 from vieux/buffer_test_integration_line_by_line

Add --line-buffered to the integration test output filtering
Merge pull request #4315 from shykes/engine-parsejob-test

Engine: add tests for ParseJob()
enable docker run -it

Docker-DCO-1.1-Signed-off-by: Victor Vieux <victor.vieux@docker.com> (github: vieux)