Remove core.hostnames pillar

Summary:
b112ea76 introduced nodes definition, including hostnames,
so it's possible to directly get the hostname from the node
entry instead of a dedicated pillar entry.

This change partially reverts b88a308fec2d.
This change reverts 866436e489cd.

Test Plan: salt * state.apply roles/core/hostname test=True

Reviewers: dereckson

Reviewed By: dereckson

Differential Revision: https://devcentral.nasqueron.org/D1270