Add support to customize polling meters

Add support to customize polling meters

This adds a new parameter called polling_meters
to the ceilometer::agent::polling class.

If you pass a array of strings this parameter
will then populate the meters in the polling.yaml
file by inserting them in the templated polling file.

The default values that was hardcoded in the template
is still the same but has been moved to the params class
as an array of strings.

Change-Id: Iae2567b5a0f24b2aa80f9640ff10df96cd656922