Make fluentd-elasticsearch configuration more robust

Enable reconnect_on_error option so that ES plugin re-establishes
a new session to the ES cluster on errors. Also, enable buffering
to the file, so that the buffer survives container restarts.

Co-Authored-By: Michal Nasiadka <mnasiadka@gmail.com>
Co-Authored-By: Radosław Piliszek <radoslaw.piliszek@gmail.com>
Co-Authored-By: Doug Szumski <doug@stackhpc.com>
Closes-Bug: #1830724
Change-Id: Ia40685b9d4fc02194e03c8791ddeb3d29d7f07f6