Add support for Ceilometer custom polling.yaml

By default, Ceilometer uses polling.yaml as cfg_file [1][2] that defines
the limit default numbers of metrics polled. Users may want to define
their own polled metrics.

[1] https://github.com/openstack/ceilometer/blob/master/ceilometer/polling/manager.py#L54
[2] https://github.com/openstack/ceilometer/blob/master/etc/ceilometer/polling.yaml

Change-Id: Iadd627ca3c767453e71c8a0c2044f00f30055f16