Cleanup unneeded PKS logic & fix for #56

[phenixblue]

What type of PR is this?

Uncomment only one /kind <> line, hit enter to put that in a new line, and remove leading whitespace from that line:

/kind bug
/kind cleanup
/kind deprecation
/kind design
/kind documentation

/kind feature

/kind release

What this PR does / why we need it:

This cleans up pre-existing logic for handling PKS (Pivotal Kubernetes Service, aka. TKGi) where the K8s Root CA Cert was stored in a non-standard location.

Previously the Root CA Cert was read from either a PKS specific ConfigMap or from the in-cluster kubeconfig.

This change moves to reading the K8s Root CA cert from the now standardized kube-root-ca.crt ConfigMap that conformant k8s clusters will have in every namespace by default.

This simplifies the code for the init container and moves to a unified logic.

Which issue(s) this PR fixes:

Fixes #56
Fixes #69

Special notes for your reviewer:

Does this PR introduce a user-facing change?:

- Move to unified logic for detecting the k8s Root CA Cert for all conformant clusters

Additional documentation e.g., usage docs, etc.:

[phenixblue]

Looks like this needs to be k8s v1.20+ for the kube-root-ca.crt configMap to be standard.

[ilrudie]

lgtm