Provide canonical directories as pillar

Summary:
Most Linux distros use /etc while UNIX standard uses /usr/local/etc
To avoid if/elif/endif everywhere, we store these directories as pillar.

Test Plan: Tested with D924 (diff 2348)

Reviewers: dereckson

Reviewed By: dereckson

Differential Revision: https://devcentral.nasqueron.org/D925