Allow parameter sources to indicate progress

[danielmitterdorfer]

In most cases, Rally can derive progress out of the box. However, there are cases when this is not possible. Consider we want to generate a certain amount of data on the fly that we want to bulk-index. Then it is not known (by Rally) upfront how many bulk request we need for that (it depends on the size in bytes of the individual bulk requests).

For these cases, Rally should allow users to provide a new property percent_complete and use it for such cases where progress indication would otherwise not be possible.