Initial move of port mapper code into sub pkg

Docker-DCO-1.1-Signed-off-by: Michael Crosby <michael@crosbymichael.com> (github: crosbymichael)
Move port mapper tests out of core and into portmapper

Docker-DCO-1.1-Signed-off-by: Michael Crosby <michael@crosbymichael.com> (github: crosbymichael)
Merge pull request #3819 from crosbymichael/move-port-mapper

Move port mapper out of core and into network drivers