Add flag to specify node image registry

[janiskemper]

/kind feature

Describe the solution you'd like
We want to have a new flag that we can give to csctl called --node-image-registry. We want to parse this value and use it in the templating. If the user specifies << .NodeImageRegistry >>, then we would like to fill the value with the value of the flag.

The flag should be also given as flag to the plugin binaries, so that they can use it.