Add service_credentials for ceilometer-agent-central

ceilometer-agent-central will run interval_task which needs
basic config of service_credentials. However, current guide doesn't
provide such setting, so the task will fail and raise Unauthorized
exception.

This patch fixes this problem by adding sample config of
service_credentials section.

NOTE: This patch also fixes some content overflow problem in related
part.

Change-Id: I38b1d417c3aa1d7b31006045c01bc6cf759ed6d0
Closes-Bug: #1268866
Add a note for ceilometer log_dir on Ubuntu

Currently, Ceilometer services on Ubuntu Precise have not been
launched with log_dir option, all the services except ceilometer-api
cannot generate their log files.

This patch adds a note for the log_dir option in case of such problem.

Change-Id: I324093df38854ca01038c99dad148bbca4c09e71
backport: havana
Closes-Bug: #1268541
Merge "Add a note for ceilometer log_dir on Ubuntu"
Merge "Add service_credentials for ceilometer-agent-central"