The default polling interval was updated from 600 to 300 in ceilometer
itself[1] but the default value in the polling class wasn't updated.
This updates the default value in our module so that we use the value
consistent with what is maintained in ceilometer repo.
[1] fc086ba3b38683765eabcdb45613463d85cb38fa
Change-Id: I7bcbbebf093228b0885b2647d86e47e2b673e518