Simplify datacubes permissions

Summary:
Move YAML anchors defining permissions to their own block.

x- or x_ can be used for values declared only to be used as YAML aliases.

Test Plan: Load pillar file with yaml.safe_load() to check it's no-op.

Reviewers: dereckson

Reviewed By: dereckson

Differential Revision: https://devcentral.nasqueron.org/D3470