Update fluentd plugin to use the new push endpoint

[joe-elliott]

The fluentd plugin is still using the old formats and push path at /api/prom/push update it to use the new json format and new path /loki/api/v1/push

Documentation of new endpoint:
https://github.com/grafana/loki/blob/master/docs/api.md#post-lokiapiv1push

[stale]

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