Update dockerctl copy to work with docker 1.8+
Update dockerctl copy to work with docker 1.8+

With CentOS7, we can no longer copy directly via devicemapper. Docker
1.8+ provides a cp function which can be used instead. This change
updates the dockerctl script to leverage the docker cp ability rather
than try and copy files directly via a devicemapper mount when docker
1.8+ is present. It will properly fall back to the devicemapper method
if docker is less than 1.8.

Change-Id: I8e81dd9e9d7eed76fea77a60ee602bef4925de20
Closes-Bug: #1524331

stackforge/fuel-library,xarses/fuel-library,xarses/fuel-library,xarses/fuel-library,stackforge/fuel-library,stackforge/fuel-library,xarses/fuel-library,stackforge/fuel-library