Add only-platforms

This is a new task-level property to allow the skipping of tasks on platforms other than the ones specified
4 jobs for pull/29/head
latest
Status Job ID Name Coverage
  Build
passed #3478
autoscale-linux
linux

00:00:51

passed #3479
openswift atbuild
osx

00:00:07

passed #3474
autoscale-linux
linux

00:00:41

passed #3475
openswift atbuild
osx

00:00:08