Support bypassing Monasca Log API for control plane logs

This change allows a user to forward control plane logs
directly to Elasticsearch from Fluentd, rather than via
the Monasca Log API when Monasca is enabled. The Monasca
Log API can continue to handle tenant logs.

For many use cases this is simpler, reduces resource
consumption and helps to decouple control plane logging
services from tenant logging services.

It may not always be desired, so is optional and off by
default.

Change-Id: I195e8e4b73ca8f573737355908eb30a3ef13b0d6