kubectl needs support for dcos EE authentication

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• kubectl subcommand needs to fetch token from local dcos.toml file
• kubectl binary needs to send Authorization: header for dcos EE compat

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tentative plan is this:

• create a dcos-kubectl binary, copy/paste of k8s/cmd/kubectl, except:
• invokes kubectl/cmd/util.NewFactory with a custom config that:
  • provides SSL information
  • provides a WrapTransport func that injects the DCOS token into the request headers

[jdef]

custom k8sm-kubectl binary: kubernetes/kubernetes#21527

[jdef]

proposed a plugin-based approach for kubectl: kubernetes/kubernetes#21553

• awaiting greenlight

[jdef]

upstream likely going this way: kubernetes/kubernetes#23066