New feature : 'chance_variable' attribute

[nj-apps]

The purpose of this new feature is to use a double variable to define set the probability.

example :

<nop>
    <action>
        <assign assign_to="pExec" variable="hangup" />
        <multiply assign_to="pExec" variable="errors" />  <!-- $pExec = $hangup * $errors -->
    </action>
</nop>

<recv response="180" chance_variable="pExec" next="2"/>