Introducing Project Data Configuration file into GPT v.2.0.0

Closes #245 (closed)

This MR extrapolates the large project config out from environment config file. With this change we simplify the environment configuration file structure and still leaving the ability to run performance tests against custom large project. Additionally we change gitaly_nodes to storage_nodes and restores project_name in GPT Data generator.

New structure for env config file:

{
  "environment": {
    "name": "localhost",
    "url": "http://localhost",
    "config": {
      "latency": "0",
      "repo_storage": "gitaly"
    },
    "storage_nodes": ["default"]
  },
  "gpt_data": {
    "large_project": "gitlabhq.json",
    "many_groups_and_projects": {
      "root_group": "gpt",
      "group": "many_groups_and_projects",
      "subgroups": 5,
      "subgroup_prefix": "gpt-subgroup-",
      "projects": 5,
      "project_prefix": "gpt-project-"
    }
  }
}

where large_project - is the name of the large project data file under k6/projects or path to user's custom project data file.

TODO:

• Add reasoning behind switching to opinionated data
• Describe how to test against custom project in docs, it should have a custom -> it should have docker command with projects mount