Use stepUp command instead of priorities as a way to designate primary nodes in tags.js

The tags.js test currently uses priorities as a way to get certain nodes to step up as primary in different points in the test. This makes the initial startup and configuration phase unstable, since it can cause an existing, lower priority primary to step down while another node is syncing from it. We can replace this technique by using replSetStepUp to force nodes to become primary throughout the test, and remove the uneven priorities between nodes.