Pin influxdb to 1.8 or update invocation

[pburkindine]

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.