Introduce events in order to make it possible to see simple log message with...

Introduce events to display simple log message with kubectl describe execution

Looks like this:

Events:
  Type     Reason                        Age    From                 Message
  ----     ------                        ----   ----                 -------
  WARNING  Start experiment failed       5m22s  theodolite-operator  load: 25000, resources: 1
  WARNING  Stop experiment failed        5m21s  theodolite-operator  Teardown failed: Failed to create new KafkaAdminClient

At the moment events are created when an experiment is started or stopped or when the execution fails.

Closes #245 (closed)