Support configuration of custom fluentd filters

As an operator, I want to be able to configure the fluentd log collector with custom filters, so that I can apply custom transformations to the log events processed by fluentd.

Kolla-ansible deploys the fluentd log collector on all hosts in order to collect logs from the containerised services and forward them to a log aggregation service. Logs are processed, and the existing kolla-ansible fluentd config adds tags for things such as log severity, hostname, etc. In some cases we may wish to apply different tags to log events, for example to conform to a site log event format. Fluentd filters provide a mechanism for doing this.

The proposed change would allow an operator to drop fluentd filter configuration templates into a directory on the control node, e.g. /etc/kolla/config/fluentd/filter/, and have these templated and distributed to the /etc/td-agent/filter/ directory in the fluentd containers.