execdriver: Make GetPidsForContainer() a driver call

The current implementation is lxc specific.

Docker-DCO-1.1-Signed-off-by: Alexander Larsson <alexl@redhat.com> (github: alexlarsson)
Initial move of port mapper code into sub pkg

Docker-DCO-1.1-Signed-off-by: Michael Crosby <michael@crosbymichael.com> (github: crosbymichael)
remove enableCors from the config and move it as a job arg

Docker-DCO-1.1-Signed-off-by: Victor Vieux <victor.vieux@docker.com> (github: vieux)
Move port mapper tests out of core and into portmapper

Docker-DCO-1.1-Signed-off-by: Michael Crosby <michael@crosbymichael.com> (github: crosbymichael)
move warnings to the job

Docker-DCO-1.1-Signed-off-by: Victor Vieux <victor.vieux@docker.com> (github: vieux)
Merge pull request #3820 from vieux/remove_runtime_from_api

Remove runtime from api
Merge pull request #3808 from alexlarsson/execdriver-get-pids-for-container

execdriver: Make GetPidsForContainer() a driver call
Merge pull request #3819 from crosbymichael/move-port-mapper

Move port mapper out of core and into network drivers