If a volume driver takes an excessively long amount
of time to return volume stats info, the only indication
if this is a generic "Function outlasted interval" message
that doesn't really explain what is going on.
Introduce a specific log message when this happens so
that this situation is more clear for someone looking
at cinder-volume logs.
The message is set to trigger if the driver takes more
than half of the stats polling interval to complete.