execdriver flag for docker daemon

  like the storage-driver flag, this implements a flag for choosing the
  execdriver to be used, defaulting to lxc.

Docker-DCO-1.1-Signed-off-by: Vincent Batts <vbatts@redhat.com> (github: vbatts)
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)
Remove trailing slash from host parameter in docker init.

(This causes errors on Docker 0.8)

Docker-DCO-1.1-Signed-off-by: Morgante Pell <morgante.pell@morgante.net> (github: morgante)
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)
api/common.go: code shared by the server and client implementation of the Docker remote api.

Docker-DCO-1.1-Signed-off-by: Solomon Hykes <solomon@docker.com> (github: shykes)
Engine: cleanup side effects between tests

Docker-DCO-1.1-Signed-off-by: Solomon Hykes <solomon@docker.com> (github: shykes)
Engine: builtin command 'commands' returns a list of registered commands

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)
Use the cpu cgroup subsystem instead of memory because its non optional
Docker-DCO-1.1-Signed-off-by: Michael Crosby <michael@crosbymichael.com> (github: crosbymichael)
Merge pull request #4312 from shykes/engine-error-reporting

Engine: clearer job error reporting
add --line-buffered

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

Docker-DCO-1.1-Signed-off-by: Victor Vieux <victor.vieux@docker.com> (github: vieux)
Merge pull request #4313 from shykes/api-common

api/common.go
Merge pull request #4309 from crosbymichael/change-cgroup-subsystem

Use the cpu cgroup subsystem instead of memory because its non optional
Merge pull request #4296 from morgante/patch-1

Remove trailing slash from --host parameter in contrib/vagrant-docker/README.md
Merge pull request #4189 from vbatts/vbatts-execdriver_flag

execdriver flag for docker daemon