Fix: Possibility of using None as a callable
Fix: Possibility of using None as a callable

The creation of the perf instances was using a reference to a class
that would not be set in the default case. It now defaults
appropriately.

Signed-off-by: Kevin Conway <3473c1f185ca03eadc40ad288d84425b54fd7d57@gmail.com>

rackerlabs/PyPerf,rackerlabs/PyPerf,kevinconway/PyPerf,kevinconway/PyPerf,kevinconway/PyPerf,rackerlabs/PyPerf