Renamed docker_registry_default_port flag

The docker_registry_default_port flag has been renamed
registry_default_port to reflect that namespacing is
already provided via the context of the "docker" group
that it appears in.

Change-Id: Ia26f747a67b36fb96f72d3cc98016342e124b152
Closes-Bug: #1249441
Merge "Renamed docker_registry_default_port flag"