Adds a --nocommit arg to the update_index management command.

This is helpful when doing bulk indexing and you'd rather have the backend handle commits, not the management command.  We had an instance where the amount of hard commits passed by update_index during a bulk index was causing stability issues with our solr cluster that was already doing soft and hard commits often.

I attempted to add this flag to clear_index and the --remove option of update_index, but it seems that solr was not respecting commit=false for those two code paths.  I can look into it some more, but wasn't able to find a solution.