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