sign tarballs and compiled slugs

[Cryptophobia]

From @arschles on January 29, 2016 20:20

Ref deis/workflow#324

Copied from original issue: deis/builder#138

[Cryptophobia]

From @Joshua-Anderson on September 8, 2016 20:26

The relevant issue is now deis/controller#324, since repository was renamed and reused.