Move docker specific options into a group.

There is only one example, but we're moving all hypervisor specific
options into groups.

DocImpact: rename docker_registry_default_port to
registry_default_port in the docker group.

Implements blueprint: docker-opts-group

Change-Id: Ic653e8a610876c40a715bff1ec48173d3bdb212e
Merge "Move docker specific options into a group."