[Packaging] GPG sign downloaded artifacts

[asfimport]

Reporter: Krisztian Szucs / @kszucs
Assignee: Krisztian Szucs / @kszucs

_{Note: This issue was originally created as ARROW-2718. Please see the migration documentation for further details.}

[asfimport]

Krisztian Szucs / @kszucs:
PR here #2162

[asfimport]

Wes McKinney / @wesm:
Resolved in bd1c332