Move platform-matrix to common and add an extra common layer for all jobs to define common variables

[safern]

This will allow use to use platform-matrix to fan out different architectures and then in xplat-setup we can start having shared stuff like directory locations, artifacts conventions, extensions, etc.

This is the first step, wanted to keep the PR as small as possible. More cleanup will be done afterwards.

[ViktorHofer]

This is the first step, wanted to keep the PR as small as possible. More cleanup will be done afterwards.

I assume that you will change the libraries yml files to depend on the platform-matrix as well? It's a bit confusing that you already add support for it here but don't consume it. It's hard to see if the matrix is correct for the libraries. Would it be feasible to also consume it as part of this PR?

[ViktorHofer]

I assumed you were adding the support for libraries in this PR already because of the "corefx" option. If this is just code-cleanup then this looks good. I won't review the platform matrix then and wait for the subsequent PR.

[safern]

I assumed you were adding the support for libraries in this PR already because of the "corefx" option. If this is just code-cleanup then this looks good. I won't review the platform matrix then and wait for the subsequent PR.

Yes that bit is confusing. This is a step towards getting platform-matrix closer to a point where it can be consumed from the libraries pipelines. Also, in order to include that consumption I would need to change all the libraries logic on how we expand our matrix and this PR would be huge and easy to miss any possible break.

I have a draft PR to get the libraries build ready to consume platform-matrix if you're curious: #71

Thanks for the review and sorry for the confusion, should've been more explicit on the description.

[ViktorHofer]

No problem at all.

[jashook]

lgtm thank you!

[safern]

763d47a is fixing the TODOs:

enabling RedHat6 helix queues in the public and also it is enabling Nano testing and moving the existing Nano queue to use the 1809 docker image.

[safern]

Merging as CI is green except but the crossgen-comparison job which is a known issue being investigated by @jashook