Make composite tasks extensible

[danielmitterdorfer]

Rally is currently strict what operation-types are allowed in a composite context because not every operation would work properly out of the box. However, it is also not possible to use custom runners in a composite context. We should therefore add a mechanism for custom runners to indicate whether they support composite contexts.

[pquentin]

We should probably do that for built-in runners, too, because maintaining a list is brittle anyway. See #1391 for context.