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)
help contributors be more productive

certainly saved me time after I found this in hack/make.sh :)

Docker-DCO-1.1-Signed-off-by: Tim Ruffles <oi@truffles.me.uk> (github: github_handle)
Merge pull request #4314 from shykes/engine-commands

Engine: builtin command 'commands' returns a list of registered commands
Convert api/server.go to also use the handy user package

Docker-DCO-1.1-Signed-off-by: Andrew Page <admwiggin@gmail.com> (github: tianon)
Merge pull request #4335 from tianon/what-the-etc-group

Convert api/server.go to also use the handy user package
Merge pull request #4323 from timruffles/patch-2

help contributors be more productive
Update mkimage-yum to not read /etc/redhat-release from the host

Docker-DCO-1.1-Signed-off-by: Andrew Page <admwiggin@gmail.com> (github: tianon)
Merge pull request #4348 from tianon/fix-yum-tagging

Update mkimage-yum to not read /etc/redhat-release from the host
Add https cert and key options to docker

Docker-DCO-1.1-Signed-off-by: Colin L. Rice <colin@daedrum.net> (github: c00w)
Add https example

Docker-DCO-1.1-Signed-off-by: Colin L. Rice <colin@daedrum.net> (github: c00w)
Simplified https example

The prior example wasn't working for me.

Docker-DCO-1.1-Signed-off-by: Johannes 'fish' Ziemke <github@freigeist.org> (github: discordianfish)
Refactor client methods to stop code duplication

Docker-DCO-1.1-Signed-off-by: Johannes 'fish' Ziemke <github@freigeist.org> (github: discordianfish)
Add tls client auth, docs and tests

Docker-DCO-1.1-Signed-off-by: Johannes 'fish' Ziemke <github@freigeist.org> (github: discordianfish)