Everywhere I've seen shamans, they've been nomads by their living habits
not because they are warriors. Here it nomad background seem to only
refer to the warrior background, not the living habits and behaviour of
the nomads.
In short, I don't see why "shamans" should have nomad background. As far
as I know, they'd be utterly shitty spellcasters no matter what (try
nomad caster in hardcore

or if their spellcasting abilities would
be boosted a lot to compensate for the background, they would be
too good tanks/spellcasters.. and I don't remember any warrior shamans
from what I've read either.. not that someone couldn't have come up with one.
Dat's it.. Smoken