Stream and release metadata signing

[bgilbert]

The stream and release metadata are meant for automatic/machine consumption, so we should sign them (and address key management).

For the moment, we are ensuring via TLS that those cannot be tampered on the wire.
Integrity of downloadable blobs (ostree commits, image artifacts) is guaranteed by direct signatures on such objects.
I think the only remaining case to cover is an overall infrastructure hijack, where somebody is able to reroute or manipulate our bucket and inject forged manifests that way. That would still not be a problem for installed machines, but may prevent new installations and auto-upgrades.

/cc @dustymabe

Originally posted by @lucab in #98 (comment)

[jlebon]

Related on the cosa side: coreos/coreos-assembler#268

[cgwalters]

My opinion on this is TLS pinning.