"pillar_merge_lists: false" support

[bobrik]

Salt itself has pillar_merge_lists setting:

• https://docs.saltstack.com/en/latest/ref/configuration/master.html#pillar-merge-lists

It defaults to false, because it's not clear how to order lists on merge. This seems like a sane default.

Pillarstack doesn't respect this setting, causing surprises when you bring assumptions from regular pillar merging to pillarstack. It seems reasonable to have regular pillars and pillarstack to look as similar as possible.