replSetFreeze command run by stepdown thread may fail when server is already primary

As part of the changes to address SERVER-35383 and based on this comment from SERVER-35124, the stepdown thread in resmoke.py runs the {replSetFreeze: 0} command to make the former primary electable in the next round of stepdowns. Since the primary is only stepped down for 10 seconds (by default), it is possible for enough time to have passed for the primary to have tried to step back up on its own before the {replSetFreeze: 0} command is run.

We either need to handle the OperationFailure: cannot freeze node when primary or running for election. state: Primary exception or prevent it from occurring.