Remove all usages of rkt

[iaguis]

rkt is not maintained anymore and keeping it is a security risk. We should remove it and replace it with something with equivalent functionality.

[invidian]

From #811

@alban had a branch for this that can serve as a reference.

[vbatts]

Also, rkt will be deprecated from flatcar 😸

[arianvp]

Is this actually done? Lokomotive documentation mentions rkt still in many places; including in the update instructions:

https://kinvolk.io/docs/lokomotive/0.6/how-to-guides/update-bootstrap-kubelet/

[invidian]

Perhaps we could update the upgrade guide in 0.7 to not mention rkt anymore, we should've done it in 0.6.

Edit: created #1360.