ceilometer: add separate polling configuration file

[aagate]

In Ocata use of pipeline.yaml for polling by ceilometer polling agent was deprecated. Instead it is recommended to add a separate polling section in ceilometer config file and set cfg_file to a new file called polling.yaml (See [1] [2] Ocata and Pike release notes)

This is preferred method because it allows to define a set of meters that can be polled which is different from meters that are collected via notifications.

The structure of the polling.yaml is similar to pipeline.yaml but only requires fields necessary for polling that is interval (polling interval) and list of patterns for source meters.

[1] Ceilometer Ocata Release Notes: https://docs.openstack.org/releasenotes/ceilometer/ocata.html

[2] Ceilometer Pike Release Notes: https://docs.openstack.org/releasenotes/ceilometer/pike.html

[rsalevsky]

Needs a rebase.

[sjamgade]

or may be add a boolean accessible in raw mode to overwrite the file or not.