HA: Tests for Mistral in multi-node mode

Now:

By design, all Mistral tiers (api, engine, executor) can scale. However, we don't have any automatic tests to see if Mistral works correctly in multi-node
mode. Mostly it's related to engine and api because they work with DB and hence there might be concurrent issues related with DB transactions.

The goal:

Create a suite of high-load tests to run Mistral in multi-node mode. These tests should prove Mistral stable in this mode and show how different number of nodes affects performance, whether we have a linear scalability (or close to it), or not. On what scenarious scalability is close to linear and what
scenarios it is not.

Solution ideas:

Design a suite of tests including:
 * Workflow with high parallelism
 * Sequential workflow
 * Workflows with joins