Add support for different docker images for a service type

[ccheetham]

Currently, a service type maps to a single docker image name. That name is expected to resolve to a docker image for both Linux and Windows docker containers.

However, in some cases it'd be easier for docker image construction if a service type could map to different image names depending on the container OS.

A specific example are Steeltoe Redis Server docker images:

• Linux -> steeltoeoss/redis-server:4.0.11-linux
• Windows -> steeltoeoss/redis-server:3.0.504-windows

[cf-gitbot]

We have created an issue in Pivotal Tracker to manage this:

https://www.pivotaltracker.com/story/show/160778835

The labels on this github issue will be updated when the story is started.