Pass Kubelet certificate CA bundle as an argument

[qdii]

What would you like to be added:

A new argument --kubelet-certificate-authority-text which allows the CA to be passed as an argument instead of as a path.

Why is this needed:

The current way to feed a CA to metrics-server is to use --kubelet-certificate-authority. This requires the certificate to be present in the metrics-server pod, which isn't easy to do in a persistent way using the Helm chart (maybe we could use extraVolumeMounts and a ad-hoc ConfigMap created for this purpose).

These additional steps makes securing the setup a little bit involved. The ability to simply pass the certificate would ease this.

/kind feature

[dgrisonnet]

/triage accepted /assign @serathius

[csauoss]

@dgrisonnet I would like to give this a try if it no one has taken it up. Thanks!

[serathius]

/assign @csauoss

[csauoss]

@serathius @dgrisonnet when you get a chance, can you please review #1516?