This change adds enable_fluentd option and enables some other log shippers
to be integrated. When enable_fluentd is "no", syslog server is also disabled.
Then, this change also adds syslog parameters to use a syslog server
prepared by users.

Change-Id: I7c83ef7fe30a6b9ab7385bcee953ad07e96b0a83
Implements: blueprint fluentd-enable-option