Add 'netlab_skip_module' node variable that will tell 'netlab initial' not to configure that module

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Sometimes (usually during development or when building a training lab) we want to skip module configuration for a particular node. The "netlab_skip_module" node attribute (a list of modules) could provide this functionality.

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This is probably best implemented with a refactoring of the clab deploy_node_config into deploy_module and moving the bulk of the functionality either into cli/initial/deploy.py or shared provider code. It makes no sense to have all this functionality (plus #2916 / #2917) in clab code (and potentially later in libvirt code)