Set the BulkFlushMaxSize on elasticsearch sink
The staging pipeline blew up with 413 Request Entity Too Large
.
It looks like, by default, the sink isn't respecting elasticsearch's
100MB limit on http entities. Set it to 80MB which should be plenty
large, while also less than the server side limit.
Change-Id: Ia97f31399a258571bf4d28cd3c7d90722ce6e474