Configure Flake8 linter for Arcanist

Summary:
Allow to specify pillar and other Salt dictionaries as builtins,
so flake8 won't complain anymore those names don't exist.

Lint also the .sls #!py files, as they are written in Python too.

This currently needs to use Nasqueron version of arcanist, production branch:
https://github.com/nasqueron/arcanist/commit/15aa482eacd90fdbd238bc8b7a17c4bbba8a0bdd

Test Plan: arc lint

Reviewers: dereckson

Reviewed By: dereckson

Differential Revision: https://devcentral.nasqueron.org/D2795