[Feature request] allow overriding of locust test pods

[namiyousef]

Is your feature request related to a problem? Please describe.
For our tests, we would like to use custom packages, but the operator doesn't allow the overriding of the image used to spin up the locust load generation pods.

Describe the solution you'd like
Configurable parameter when defining the locust CR to allow the use of a custom image to spin up the load generation pods