Create simple LoadQoSController which will evict revocable executors when system load is too high.

It would be nice to have simple QoS Controller module out-of-box in Mesos (Similar to the FixedResourceEstimator).
As a result we will:

• enable simple QoS assurance and encourage users to use oversubscription modules.
• present example QoS module for new developers.

The idea is to implement LoadQoSController which will monitor system load and evict all revocable executors when the load is above configured threshold.
In implementation, it will support two configurable thresholds: system load average 5min and 15min. System load 1 min will be ignored, since it could be misleading signal when temporary fluctuations appears.

We could also add description to the oversubscription.md doc.