Provide a home_prefix pillar that is used as prefix for home location

[arthurzenika]

It would be nice to be able to, instead of configuring home (with a duplicated login info), to have a home_prefix pillar that is used instead of the default value "/home"

[0xf10e]

I think there was some discussion about formula-level configuration via pillar-data somewhere. I think we
ended up with pillar[xyz_formula:option] or something.

Following that pattern we could just use pillar[users_formula:home_prefix] for your idea.