Add polling.yaml to managed configs

This change adds polling.yaml to the list of managed config files, for
Queens and above.  Two new config items are added alongside the
template, to take into account a choice between the two polling.yaml
files supplied with the Ceilometer source, one which has a small list of
pollsters and the other which has all pollsters.  Additionally the
change adds the option to set the polling interval.

Change-Id: Ifff72870cf79bea23dbd21586857dd832c9e1405
Closes-Bug: 1785933
Closes-Bug: 1652848