apply_batch_only_goes_forward.js should be configured to use 2 electable nodes instead of 3

After restarting the initial primary node with a modified minValid, one of the remaining electable nodes will step up as primary. Occasional network slowness may result in a primary change between the 2 remaining electable nodes. We can configure this test to use 2 electable nodes (third node will be an arbiter) to eliminate the possibility of an election during this phase of the test.