Add integration tests

[fede1024]

It would be useful to have integration tests.x

Some possible test scenarios are:

• Produce some data into Kafka, wait for metrics to refresh, then check rate, topic size.
• Create a topic with a specific layout (partitions/replicas), and verify that the topology reported by the API matches the expected one.
• Verify that the topic tailer returns the messages as they are being produced.
• Create a consumer group, commit some offsets, and verify that the API reports the correct values.
• Delete a topic, check that the topic disappears from the API and from the cache.

This could be easily achieved by running kafka-view in a dockerized environment using docker-compose, similarly to what is done for the integration tests of rdkafka.