Make it possible to disable polling for bandwidth usage

Bandwidth usage is only supported by some hypervisor drivers, but
the period task always runs and asks conductor for a list of
instances before it gets a NotImplementedError from the virt driver.

This change allows this to be disabled by setting bandwidth_poll_interval
to 0 which is consistent with other periodic task interval settings,
avoiding the wasted conductor call.

It also will automatically disable bandwidth polling if the driver raises
NotImplemented.

Change-Id: I2ac9c967c5ceafffc39a0a372146c762891a08b8
Merge "Make it possible to disable polling for bandwidth usage"