Fix Mongo task to strip dates

As of Mongo 2.4, the JSON response includes a Timestamp(int, int)
value to indicate optime. That doesn't parse as JSON, so we need
to regex it out in the same way we do for ISODate().
Improve printing of Mongo replicaSet status

Convert to use the Python 3 printing format. Make the table line
up and be a bit prettier.
Improve Mongo step down task

This task was previously checking to see whether Mongo returned JSON
of the form {ok: 1}. This is impossible, because when running --eval
from the command line the current session will be disconnected with
the following warnings and the exit code 252:

Thu Apr 10 13:36:20.171 DBClientCursor::init call() failed
Thu Apr 10 13:36:20.172 Error: error doing query: failed at src/mongo/shell/query.js:78

Instead, run the command with warn_only enabled, and test for the
exit code 252 when printing a response to the user.
Merge pull request #44 from alphagov/fix-mongo-status

Fix and improve Mongo commands