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.

providers/azure/README.md

@@ -320,7 +320,8 @@ spec:
   dnsPolicy: ClusterFirst
   nodeName: virtual-kubelet-myconnector-linux
   tolerations:
-  - key: azure.com/aci
+  - key: virtual-kubelet.io/provider
+    value: azure
     effect: NoSchedule
 ```
 
@@ -328,7 +329,8 @@ Notice that Virtual-Kubelet nodes are tainted by default to avoid unexpected pod
 
 ```
   tolerations:
-  - key: azure.com/aci
+  - key: virtual-kubelet.io/provider
+    value: azure
     effect: NoSchedule
 ```