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)
Add "docker create" support

This exposes the already existing "create container" operation.  It is
very similar to "docker run -d" except it doesn't actually start the
container, but just prepares it. It can then be manually started using
"docker start" at any point.

Docker-DCO-1.1-Signed-off-by: Alexander Larsson <alexl@redhat.com> (github: alexlarsson)