Set a performance test configuration property.
A property may have a single value, or a sequence of values, separated by ":".
If a sequence of values is specified, each time a new round starts,
the next (cyclic) value is taken.
Other side effects: none.
Takes mandatory param: "name,value" pair.
Perform the task once (ignoring repetitions specification)
Return number of work items done by this task.
For indexing that can be number of docs added.
For warming that can be number of scanned items, etc.