Actual Fluentd config doesn't manage multiline log pattern like:

2017-08-16 21:41:03.097 7 WARNING oslo_config.cfg [-] Option "firewall_driver" from group "DEFAULT" is deprecated for removal (
nova-network is deprecated, as are any related configuration options.
). Its value may be silently ignored in the future.

This log will generate 1 Fluentd event for each line (in previous
example 3 events).
This is really a wrong behaviour and made Kibana interface not friendly.
This patch uses a multiline parser.

Change-Id: Iee337645d1dc42876ec79be08ee3715f11a53d87
Closes-Bug: #1711399