Add a dockercfg_path parameter to the Mesos scheduler (the main purpose is to handle private repos auth).

This is implemented by copying the dockercfg file in the executor work directory.

hack/verify-flags/known-flags.txt

@@ -60,6 +60,7 @@ dest-file
 disable-filter
 docker-endpoint
 docker-exec-handler
+dockercfg-path
 driver-port
 dry-run
 duration-sec