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
(cherry picked from commit 93688b5133fcd003eeb046d5dfafebbc86ee5adb)