Introduce ceilometer:config to manage custom options

This ceilometer::config is aim to use ceilometer_config resource
to manage custom configurations in ceilometer.conf.

This will make end user easy to add their own custom options
in Hiera data.

Fully implements blueprint ceilometer-custom-config

Change-Id: I82b38ca26d4d99471a74b2c116e4c4133262187c
Merge "Introduce ceilometer:config to manage custom options"