Switch salt unit pillar configuration to nodes

Summary:
b112ea76a1a7b has introduced nodes definition, with a roles property.
This change uses it instead of legacy salt.roles pillar.

Test Plan: salt-call --local state.apply roles/core/salt

Reviewers: dereckson

Reviewed By: dereckson

Differential Revision: https://devcentral.nasqueron.org/D1075