add 'afr' for user to set wrok acceptable failure ratio in configure file

add 'afr' for users to set different acceptable failure ratio for \<work\> in a workload configure file.
The unit is samples per one million, default is 0.
eg.  \<work name="work12" workers="16" runtime="300" afr="500"\> 
it means the acceptable failure ratio for this \<work\> is 0.0005 (= 500 / 1000000)