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.
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