Fix clab device specific kernel module loading

[jbemmel]

The current devices only use kmods: (empty) to specify they need system default kernel modules loaded

• Cumulus NVUE needs ebtables loaded too, regardless of which modules are used (modeled as depending on 'initial')
• Linux nodes don't need any (don't support any modules that would require additional kmods); ndata.module becomes {} for hosts

The original code wasn't actually using ddata.kmods, but because none of the current devices had any value for it that didn't cause any issues