Configurable test order optimizer strategy

[OndraM]

Currently, we have MaxTotalDelayStrategy class, which takes care of calculating test order.

Thanks to configuration file introduced in Steward 2.2, the class which handles the test order calculation could be made configurable, to allow implementation of some custom strategies (one idea was strategy which would take in account not only tests delay but also average tests length, so the longest running tests could be started first).

The class must implement OptimizeOrderInterface. The whole logic (and option validation etc.) would be very similar to the capabilities_resolver option (which is already present in Steward 2.2).