[Locust] Support a way to install additional Python packages

[anh-nguyen-simplisafe]

I use some additional Python packages in my loadtest such as redis, pymongo, pika ... It would be nice to support the install command as an input that executes as part of the setup process. Currently I work around by adding the install commands in the entrypoint shell scripts but that is every manual and I am unable to publish the code for the team use.

Or is there a way to do it that I missed?