Add default provider taint and taint configuration options

This allows for more specificity when setting taint tolerations for
workloads. Three new env variables are introduced:

VKUBELET_TAINT_KEY (defaults to `virtual-kubelet.io/provider`)
VKUBELET_TAINT_VALUE (defaults to provider name)
VKUBELET_TAINT_EFFECT (defaults to `NoSchedule`)

BREAKING CHANGES:
- The default taint key of `azure.com/aci` is now
  `virtual-kubelet.io/provider`.
- Specifying a custom taint key is now done via an environment variable
  rather than the `--taint` command line flag.

examples/nginx-pod.yaml

@@ -19,5 +19,5 @@ spec:
     beta.kubernetes.io/os: linux
     type: virtual-kubelet
   tolerations:
-  - key: azure.com/aci
-    effect: NoSchedule
+  - key: virtual-kubelet.io/provider
+    operator: Exists