Add automatic download of kubectl

[afbjorklund]

Currently we are pushing the install of kubectl to the user, with varying results (and long docs).

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kubectl could not be found on your path. kubectl is a requirement for using minikube
To install kubectl, please run the following:

curl -Lo kubectl https://storage.googleapis.com/kubernetes-release/release/v1.13.3/bin/linux/amd64/kubectl && chmod +x kubectl && sudo cp kubectl /usr/local/bin/ && rm kubectl

To disable this message, run the following:

minikube config set WantKubectlDownloadMsg false
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It would be more user-friendly to offer a wrapper to download and start the proper os/arch binary:

$ minikube kubectl version
Client Version: version.Info{Major:"1", Minor:"13", GitVersion:"v1.13.3", GitCommit:"721bfa751924da8d1680787490c54b9179b1fed0", GitTreeState:"clean", BuildDate:"2019-02-01T20:08:12Z", GoVersion:"go1.11.5", Compiler:"gc", Platform:"linux/amd64"}
Server Version: version.Info{Major:"1", Minor:"13", GitVersion:"v1.13.3", GitCommit:"721bfa751924da8d1680787490c54b9179b1fed0", GitTreeState:"clean", BuildDate:"2019-02-01T20:00:57Z", GoVersion:"go1.11.5", Compiler:"gc", Platform:"linux/amd64"}

This also picks the right version (eventually), and offers to use more than one version.

If the user prefers to install and use their own kubectl, they just avoid this command...

[afbjorklund]

@tstromberg

[afbjorklund]

Note that I didn't have to do anything above, just wait a bit - while the kubectldownload happened.

i.e. I didn't have "kubectl" installed anywhere when I ran the command, yet it completed successfully.

[RA489]

@afbjorklund Is it still open ? if yes i would like to help.

[afbjorklund]

@RA489 : I want to make it download the current kubernetes version (instead of the default), but other than that I think #3646 is more or less what is needed ? Perhaps clean up the duplicated code a bit, test on Win.

[afbjorklund]

Updated the implementation, now it has some better download information and exit code...
Also respecting the current profile kubernetes version (if any) and removed code duplication.