Avoid duplication of Roslyn versions

[chcosta]

There are a couple of places where Roslyn versions are duplicated. We should look to avoid the duplication where possible. See #947

Can we avoid having to hard code this version and package name here by moving the logic to the init-tools scripts and just copy the props files into the ToolsRuntimePath similar to the portable targets?

I really don't like seeing this version copied multiple places :( Do we have any strategies to try and avoid this duplication?

[ViktorHofer]

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