(For "Integrate and fully support OSX Signing in taskcluster")
Written as a mozharness script rather than using bare ./mach command because we need to download the upstream artifact
and because we need to download artifacts from tooltool to do the packing back into a .dmg. Future ideal would be to get
rid of the mozharness script and use JUST ./mach.
This is using the ./mach repackage code being created in Bug 1347576. Taking a signed tarball from a dmg supported with
Bug 1346015, and the taskgraph work to schedule this is in Bug 1318505.
MozReview-Commit-ID: rv9l285HKC
