save and restore gdt variables in gdt-kube tests

[jaypipes]

Adds the gdt variable system functionality to the gdt-kube plugin, allowing test authors to define variables that should be populated from inspecting the results of the kube.get output and extracting an element using a JSONPath expression.

You can mix and match gdt test specs and the gdt variable system to pass values between test specs, allowing for a really extensible testing framework, like this example shows:

name: curl-pod-ip
description: scenario showing how to create two NGinx Pods and test connectivity to internal Pod IP addresses
fixtures:
  - kind
defaults:
  kube:
    namespace: curl-pod-ip
tests:
  - name: create-server
    kube:
      create: testdata/manifests/nginx-server.yaml

  - name: get-server-pod-ip
    kube:
      get: pods/server
    assert:
      conditions:
        ready:
          status: true
    var:
      SERVER_IP:
        from: $.status.podIP

  - name: create-connect-tester
    kube:
      create: testdata/manifests/nginx-connect-test.yaml

  - name: wait-connect-test-ready
    kube:
      get: pods/connect-test
    assert:
      conditions:
        ready:
          status: true

  - name: curl-server-from-connect-tester
    exec: kubectl exec -n curl-pod-ip pods/connect-test -- curl -s -I -v $$SERVER_IP
    timeout: 2s
    assert:
      out:
        contains: "200 OK"
      # curl -v causes output to be sent to stderr that looks like this:
      # * Connected to 10.244.0.17 (10.244.0.17) port 80 (#0)
      # > GET / HTTP/1.1
      # > Host: 10.244.0.17
      # > User-Agent: curl/7.88.1
      # > Accept: */*
      # >
      # < HTTP/1.1 200 OK
      err:
        contains: "Host: $$SERVER_IP"

  - name: delete-connect-tester
    kube:
      delete: pods/connect-test

  - name: delete-server
    kube:
      delete: pods/server

Issue gdt-dev/gdt#51

[a-hilaly]

Noiiice!