server: Break out setHostConfig() from ContainerStart

This will be reused for ContainerCreate

Docker-DCO-1.1-Signed-off-by: Alexander Larsson <alexl@redhat.com> (github: alexlarsson)
api.DockerCli: Extract pullImage into separate function

This lets us reuse this code later.

Docker-DCO-1.1-Signed-off-by: Alexander Larsson <alexl@redhat.com> (github: alexlarsson)
Merge pull request #6702 from mrunalp/dev/helper_fns

Move setting hostConfig and pulling images into helper functions.