Share yml templates between runtimes

[ViktorHofer]

There's a lot of duplication between the following yml files for mono and coreclr which could be shared by moving the common stuff into the common templates folder.

• eng/pipelines/mono/templates/xplat-job.yml
• eng/pipelines/coreclr/templates/xplat-job.yml

and

• eng/pipelines/mono/templates/xplat-pipeline-job.yml
  eng/pipelines/mono/templates/xplat-pipeline-job.yml

cc @safern @akoeplinger