Setup and teardown for cloud execution

[robingustafsson]

#457 implements setup & teardown lifecycle hooks for tests. However, it doesn't implement the necessary hooks for the feature to work with cloud/distributed execution, namely:

• We need to be able to run only the setup function (and pause) and then distribute the result/return of the setup function to all nodes' export default function:

  export default function(setupData) {
      http.get(`https://${setupData.featureBranchSubdomain}.example.com/`);
  }

• We need to be able to extract the data returned by the setup() call from the node where it was run, and pass it to all other nodes' export default function. Maybe through the REST API? So, you could start k6 with something like:

  1. run k6 cloud --run-setup-then-pause script.js
  2. extract the setupData
  3. pass it to all nodes (k6 run --setup-data "..." or via REST API), and then unpause all nodes to begin execution

The intent is to merge #457 first to make it available in k6 for local execution and then implement what's outlined in this issue to gain the necessary cloud/distributed execution support.

[robingustafsson]

Also, as noted by @na-- in #457 the setup and teardown function timeouts (hardcoded to 10s for now) should be made configurable.

[na--]

Closing this because #629 should be sufficient to implement the rest of the needed changes outside of k6 itself.