Find a way to distribute pillars with the formulas

[tacerus]

Some formulas go together with pillar macros or special #!py pillars to reduce duplicated pillar code which is not feasible to implement as part of the state code.

Currently, the formulas only ship file_roots, and the pillars are managed separately in the individual downstream Salt repositories, leading to duplicated pillar code and changes not getting synchronized between all consumers of the formulas.
A way to bundle pillar_roots which seamlessly integrates with existing downstream pillar_roots/ext_pillars should be found.