qa: ignore NVMEOF_GATEWAY_DOWN in nvmeof_scalability.yaml

qa: ignore NVMEOF_GATEWAY_DOWN in nvmeof_scalability.yaml

Sometimes during scale-up/scale-down, a gateway goes in
UNAVAILABLE state (which triggers NVMEOF_GATEWAY_DOWN warning)
for a couple of seconds and self-recovers.
In this, none of the scale test asserts fail.

So NVMEOF_GATEWAY_DOWN can be ignorelist, because scale test asserts
on expected gw count and checks if all expected gws are AVAILABLE
between each iteration of scale-up/scale-down.

Fixes: https://tracker.ceph.com/issues/75179
Signed-off-by: Vallari Agrawal <vallari.agrawal@ibm.com>