Disable cobra flags after kubectl sub-command

[JoshVanL]

What this PR does / why we need it:
This fixes a problem whereby we were not passing flags or -- to kubectl correctly as they were being interpreted by croba, e.g. tarmak kubectl exec -it shell-demo -- /bin/bash
This sets all arguments after kubectl to be passed to kubectl.

fixes #477

Disable cobra interpreting flags after kubectl sub-command

/assign @simonswine

[simonswine]

I think a PR should remove the workaround code in here: https://github.com/jetstack/tarmak/blob/master/cmd/tarmak/cmd/root.go#L46

And also apply for the SSH command. A important test is if all arguments before kubectl/ssh are still parsed accordingly by tarmak

[JoshVanL]

/unassign @simonswine
/assign

[JoshVanL]

/unassign
/assign @simonswine

[simonswine]

Nice work. Esp the testing @JoshVanL

Wasn't able to break it.

[simonswine]

/lgtm

[simonswine]

/approved

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