Need a way to set services under test version

[big-andy-coates]

Currently the code defaults to latest for the version of the Docker image to use for the services under test. Ideally, users should be able to configure what version of each service they want to test.

This likely needs both an enhancement here and another in the build plugins.

Probably want to provide different strategies for determining the image versions, e.g.

• latest: (default) get the latest version of the image, i.e. current functionalty.
• from_jar: extract the version info from the filename of the jar the descriptor was loaded from.
• specific: look somewhere for specific version info.