Stop ensure_indexes running on a secondaries

[rozza]

As it will fail!

[DavidBord]

I think that a more robust solution would be something like:
if not collection.is_mongos and collection.read_preference > 1:
return
because otherwise connections to a mongos and not a mongod won't ensure indices at all

I know it is way bigger than the scope of this issue, but this is one of the moments that I would like to have a log message saying that no indices were created