Fix sensors on arista-eos

r7183 caused discovery to save all sensor oids as "aristaEntSensorThresholdHighWarning" instead of the correct .1.3.6.1.2.1.99.1.1.1.4.whatever, due to reusing the variable name $oid.

I haven't evaluated whether or not the newly-introduced code is doing the right thing with the limits - right now I just want to get fundamental sensor parsing correct.