Clean up output of mongo.step_down_primary

Stepping down a primary causes a client disconnection which results in a
252 exit code.  We can tell fabric that this is expected using
env.ok_ret_codes.  This cleans up the output greatly, since previously
run() would emit a warning for the 252 code, and then we had to print
more output explaining that the warning was okay.
Remove now-unused features of run_mongo_command

This cruft was only there for the previous usage of step_down_primary.
It isn't needed anymore.
Merge pull request #57 from alphagov/clean-mongo-step-down-output

Clean mongo.step_down_primary output