Add option to keep InfluxDB instance alive after attack cluster teardown

[rpappalax]

We'll need this in order to review data post test run. An InfluxDB node is created together with the attack cluster, but entire cluster is destroyed at end of run. In order to review the data after the run, we'll need to keep this instance around to query the data.
Long-term goal would be instead to forward (dump) the data to a collective data store like S3 or a permanent InfluxDB instance (as indicated in Issue #70)

[rpappalax]

See: Issue #70