Enhance --override-zos-tools option to allow for selected tools to override

[IgorTodorovskiIBM]

There may be cases for more granularity in --override-zos-tools. If the need comes, we could extend it to accept a comma or space delimited list of tools to add. So for example, if the file system includes all of the tools, but a user wants to include altbin/make, they can specify --override-zos-tools=make.

Related item: #766

[gngrossi]

If feasible, it would be good to provide a config file as input to the --override-zos-tools option. As the list of tools may increase over time, it would be easier to manage/use.
thanks