Expose --disable-cloud-provider feature and cloud-provider= kubelet arg

[anmazzotti]

The current implementation of DisableExternalCloudProvider is a bit confusing (sorry for that).

As discussed in this comment, it would be more explicit to have two dedicated fields in the server config:

1. disableCloudProvider to add the --disable-cloud-provider feature flag on k3s.
2. cloudProviderName to set an explicit cloud provider name.

The DisableExternalCloudProvider flag should be deprecated in the upcoming v1beta2 version.

An equivalent config would then be:

serverConfig:
   disableCloudProvider: true
   cloudProviderName: external