Seperate loop and plugin control in the kube-controller-manager.
Adding an "--external-plugin" flag to specify a plugin to load when
cloud-provider is set to "external". Flag has no effect currently
when the cloud-provider is not set to external. The expectation is
that the cloud provider and external plugin flags would go away once
all cloud providers are on stage 2 cloud-controller-manager solutions.
Managing the control loops more directly based on start up flags.
Addressing issue brought up by @wlan0
Switched to using the main node controller in CCM.
Changes to enable full NodeController to start in CCM.
Fix related tests.
Unifying some common code between KCM and CCM.
Fix related tests and comments.
Folded in feedback from @jhorwit2 and @wlan0
Automatic merge from submit-queue (batch tested with PRs 50289, 52106)
Honor --use-service-account-credentials in cloud-controller-manager
If --use-service-account-credentials is specified, the cloud controller manager should honor it
The distinction between the rootclientbuilder and the clientbuilder came from kube-controller-manager, which is responsible for running the very controllers that enable service accounts. That two-layer approach is not needed in the cloud-controller-manager.
```release-note
The `kube-cloud-controller-manager` flag `--service-account-private-key-file` was non-functional and is now deprecated.
The `kube-cloud-controller-manager` flag `--use-service-account-credentials` is now honored consistently, regardless of whether `--service-account-private-key-file` was specified.
```
