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