Since we have app.default_pool* properties, create appropriately named
pool and let those properties choose the right pool. This also means we
don't need to specify pool name in default volume config anymore
QubesOS/qubes-issues#2256
Always define those properties, always include them in volume config.
Also simplify overriding pool based on volume type defined by those:
override pool unless snap_on_start=True.
QubesOS/qubes-issues#2256
