Pin influxdb to 1.8 or update invocation

The influxdb docker image was recently updated to 2.x, and now requires additional ENV variables to be set.

https://hub.docker.com/_/influxdb

jmeter-operator was not spinning up correctly; Grafana would show "Not authorized" and not able to connect using admin:admin to influxdb. Pinning `influxdb_image` to `influxdb:1.8 ` solved the problem.

Locate the configuration entry for `influxdb_image` and inspect how the InfluxDB container is invoked. Verify the selected image starts correctly and that Grafana can connect with the documented credentials; done means jmeter-operator and its Grafana integration start without the authorization failure.

Written by the indexing model from the issue text.