Integrate workload generator

[mcopik]

Currently we only support simple invocation policies with sequential and parallel invocations. We have a first pending PR to integrate an external invoker with sinusodial pattern (#212). We should merge that and then integrate a new workload generator, such as FaaSRail.

Tasks

• Add few more patterns - linear scaling, bursty, constant workload.
• Integrate artillery into SeBS as an invocation option.
• Add artillery as a dependency for the project.
• Get HTTP trigger for selected function.
• Test extensively.