Add option to avoid test failing if traffic is not intercepted

[pablochacin]

#223 introduced a validation in the agent in order to report an error if no traffic was intercepted by an agent.

This change was introduced in order to detect misconfigurations causing the fault injection not to affect the traffic. For example, specifying the wrong target port, or selecting pods with the wrong criteria (e.g. wrong labels)

However, there are several scenarios in which an agent can receive no traffic. For example, when sending a single (or a few) request to a service backed by multiple pods. In this case, with a low error rate, some pods may not receive any traffic at all.

Therefore, it would be convenient to offer users an option to prevent this behavior.