Monasca specific config for Fluentd

This commit adds some filters which format logs so that they
can be correctly sent to the Monasca Log API by the Monasca
Fluentd plugin. In the future the Fluentd plugin could be
extended and this config could be removed.

Partially-Implements: blueprint monasca-roles
Change-Id: I87b6dfb3052d03f87349d30b66078c39d625195d