Allow reordering config repo pipelines

[arvindsv]

Attempt to implement #6042.

See documentation for:

• Controlling display order in YAML plugin
• Controlling display order in JSON plugin

How it works:

At the level of a pipeline (for instance, here), add a property called: display_order set to any integer. When the repository is parsed and the pipeline is shown in the dashboard, the order of the pipelines in each group is based on their weight, lowest to highest. Pipelines defined in XML have a weight of -1 and there is no way provided to change that.

Ideally, the format_version of the JSON file should be changed to 4, but earlier versions will also work.

Example:

{
  "format_version" : 4,
  "display_order": 11,

  "name": "allmaterials",
  "group" : "configrepo-example",
  "enable_pipeline_locking": true,
  "environment_variables": [],

  "materials": [
    {
      "url": "https://github.com/tomzo/gocd-json-config-example.git",
      "type": "git",
      "name" : "mygit",
      "destination" : "mydest"
    }
...

Todo:

• Update plugin API documentation. [See: Document display_order_weight plugin-api.go.cd#104]
• Update functional tests? [Looked in both gocd/functional-tests and gocd/ruby-functional-tests and didn't see anything relevant.]
• Update YAML plugin + documentation. [See: Add ability to control display order of pipelines on the GoCD dashboard tomzo/gocd-yaml-config-plugin#114]
• Update JSON plugin + documentation. [See: Document ability to control display order of pipelines on the GoCD dashboard tomzo/gocd-json-config-plugin#43]
• Update Groovy plugin + documentation. [See: Add support for display_order_weight to order pipelines within a gr… gocd-contrib/gocd-groovy-dsl-config-plugin#60]
• Consider: Is display_order_weight the right term to use? [Changed to display_order]
• Bump up bundled version of plugins. [See: #6043 - Bump bundled YAML config repo plugin version #6092]

Note: This PR can be merged now. This doesn't need to wait for the todos. But, if no one has concerns and this gets merged, the rest can be worked on.

Scenarios considered:

1. Target version set to 4 and display_order set to 10.
2. Target version set to 3 and display_order set to 10. Doesn't complain. Uses 10.
3. Target version set to 4 and display_order set to -10 (shows up before pipelines defined in XML).
4. Target version set to 4 and no display_order is set. Defaults to -1.
5. Different pipelines having the same weight. This works fine. The order (within those pipelines with the same weight) is indeterminate. These values are weights and not the order itself.
6. Export to JSON/YAML might not export display_order. I'm not too worried about it. I think it's ok not to. [Checked: Works in all three plugins.]

Trying it out manually:

After building using the code from this PR, adding this repository as a JSON code repo, should work: https://github.com/arvindsv/gocd_6043

Since the JSON plugin doesn't really do much validation, it will work, even without any support added to it.

Reviewing this change:

It's not very complicated.

1. A temporary weight is stored at the pipeline config level.

2. A jolt migration which is tested here takes care of defaulting the value to -1. It's just to maintain consistency. This wasn't strictly needed.

3. This is where order used to get lost, earlier.

4. This test shows how it is used in the dashboard representer, just before sending it.

Most of the rest of the code is test files, most of which consist of just some small refactoring.