Allow to provision /home for every user

Summary:
This is a follow-up for 93a72bcf.

Refactor devserver user flags

This datastructure change allows to provide a list of tasks to run,
matching a block in the userland-home unit, to do more than copy files.

Plan is to allow to bootstrap local development environments, installing
non-global software, for example with cargo install.

Test Plan:
Run on WindRiver, ensure it's no-op:

salt-call --local state.apply roles/devserver/userland-home/homefiles test=True

Reviewers: dereckson

Reviewed By: dereckson

Differential Revision: https://devcentral.nasqueron.org/D2384