Provide default opt-in and opt-out options for sidecar injection

[fawadkhaliq]

Issue #291

Description of changes
The current default MutatingWebhookConfiguration for webhook mpod.appmesh.k8s.aws was to only send requests to the injector with namespace label appmesh.k8s.aws/sidecarInjectorWebhook: enabled. This would enable sidecar injection by default for pods in the labeled namespace and only provide an option to opt-out of sidecar injection for users who would have like to disable injection on a subset of pods in a namespace.

This change will do matchExpressions on key: appmesh.k8s.aws/sidecarInjectorWebhook and values: enabled and disabled to send to request to the injector and will set the default injection based on the namespace label.

appmesh.k8s.aws/sidecarInjectorWebhook: disabled: The sidecar injector will not inject the sidecar into pods by default. Add the appmesh.k8s.aws/sidecarInjectorWebhook annotation with value enabled to the pod template spec to override the default and enable injection.
e.g.

apiVersion: v1
kind: Namespace
metadata:
  name: default-disabled
  labels:
    mesh: default-disabled
    appmesh.k8s.aws/sidecarInjectorWebhook: disabled
---
apiVersion: apps/v1
kind: Deployment
metadata:
  name: override-default-disabled
  namespace: default-disabled
spec:
  template:
    metadata:
      annotations:
        appmesh.k8s.aws/sidecarInjectorWebhook: enabled //this will override the default and inject sidecar
    spec:
      containers:
      - name: override-default-disabled
        image: tutum/curl

appmesh.k8s.aws/sidecarInjectorWebhook: enabled: The sidecar injector will inject the sidecar into pods by default. Add the appmesh.k8s.aws/sidecarInjectorWebhook annotation with value disabled to the pod template spec to override the default and disable injection.
e.g.

apiVersion: v1
kind: Namespace
metadata:
  name: default-disabled
  labels:
    mesh: default-disabled
    appmesh.k8s.aws/sidecarInjectorWebhook: enabled
---
apiVersion: apps/v1
kind: Deployment
metadata:
  name: override-default-enabled
  namespace: default-enabled
spec:
  template:
    metadata:
      annotations:
        appmesh.k8s.aws/sidecarInjectorWebhook: disabled //this will override the default and disable inject sidecar
    spec:
      containers:
      - name: override-default-enabled
        image: tutum/curl

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