ostree: silverblue manifest is now fedora-silverblue.yaml

[Dusty Mabe]

Corresponding configs change is in:
https://pagure.io/workstation-ostree-config/pull-request/115#

This will require the bodhi machines to have the hotfixed
pungi rpm with this fix: https://pagure.io/pungi/pull-request/1088
This was already hotfixed on the composer machines.

Dusty