Warmup period might be set incorrectly

[leokondrashov]

Currently, warmup period is only set in configuration, with no validation on whether this amount of time for warmup is enough. This creates a situation when the resulting measurements are useless due to warmup taking longer than it was set in the configuration.

There is a proposal of a solution: add a runtime check for the end of warmup period whether the cluster has stabilized. If it's not, we should finish execution and notify the user about the fact that set warmup period is too small for current setup to stabilize under current load. Also, it should be passed to experiment driver used for load sweeps through different traces, since failed to warmup for smaller trace means that the same will happen for a bigger load.