support set-based label selectors

[jaypipes]

Adds a flexible label selector functionality to the gdt-kube plugin.

When selecting objects from the Kubernetes API, you can use a label selector in the kube.get and kube.delete test spec actions. This label selector functionality is incredibly flexible. You can use a kubectl-style label selector string, like so:

 - name: select pods that have the "app=argo" label but do NOT have the "app=argo-rollouts" or "app=argorollouts" label
   kube:
     get:
       type: pod
       labels: app in (argo),app notin (argo-rollouts,argorollouts)

The kube.get.labels field can also be a map of string to string, which is more aligned with how gdt's YAML syntax is structured. This example selects pods that have both the app=myapp and the region=myregion label:

 - name: select pods that have BOTH the app=myapp AND region=myregion label
   kube:
     get:
       type: pod
       labels:
         app: myapp
         region: myregion

The kube.get.labels-in field is a map of string to slice of strings and gets translated into a "label IN (val1, val2)" expression. This example selects pods that have either the app=myapp label or the app=test label:

 - name: select pods that have EITHER the app=myapp OR app=test label
   kube:
     get:
       type: pod
       labels-in:
         app:
          - myapp
          - test

The kube.get.labels-not-in field is also a map of string to slice of strings and gets translated into a label NOTIN (val1, val2) expression. This example selects pods that do not have either the app=myapp or the app=test label.

 - name: select pods that have DON'T HAVE the app=myapp OR app=test label
   kube:
     get:
       type: pod
       labels-not-in:
         app:
          - myapp
          - test

You can combine the kube.get.labels, kube.get.labels-in and kube.get.labels-not-in fields to create complex querying expressions. This example selects pods that have the app=myapp label and have either the category=test or category=staging label and do not have a personal=true label:

 - name: select pods that have have an app=myapp label AND have either a category=test or category=staging label AND do not have the personal=true label
   kube:
     get:
       type: pod
       labels:
         app: myapp
       labels-in:
         category:
          - test
          - staging
       labels-not-in:
         personal:
          - true

Issue #35