Providing greater control over the loadgenerator

[Mwad22]

Feature Request

Providing greater control over the loadgenerator.

Is your feature request related to a problem?

Currently, there is not much control given to the user over the loadgenerator beyond being able to change the number of USERS via an environment variable. No simple way to control the loadgenerator, and locust web UI is specifically disabled via --headless flag at docker entrypoint (see here) so users aren't able to override it.

Describe the solution you'd like:
Providing more control over the loadgenerator via the following:

• removing the --headless and -u tags from loadgenerator's docker entry point and supporting this default behaviour via LOCUST_HEADLESS and LOCUST_USERS environment variables. This allows users to override these environment variables in case they would like to expose the Locust UI and allow users more control over the loadgenerator.
• Supporting LOCUS_HOST option. Allows vendors that want to add an ingress route to the demo on k8s to direct loadgen to go through the ingress route.

Describe alternatives you've considered.

On k8s, setting env variable USERS to 0 and using command line within the loadgenerator pod.

[puckpuck]

This was fixed in #316