Add the ability to specify ContainerInfo globally (Docker/Pluggable Containerizer)

**WIP**

I've added the ability specify a the value for `CommandInfo.ContainerInfo.image` when using the new Pluggable Containerizer code (still in development/review [r17567](https://reviews.apache.org/r/17567/)).

This implementation assumes you want to have a single container for all jenkins slaves. It might be nice to add the ability to mix slave labels + custom container images to allow jenkins jobs to use different images (one slave would only take jobs for that image, of course).