ConfigWriter: add checks for be/nd/nic params

This adds checking (in the configuration) for invalid be, nd and nic
params. The code is a bit tricky as nd params are at cluster,
nodegroup and node level, nicparams are at cluster and nic level,
whereas beparams are at cluster and instance level.

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: René Nussbaumer <rn@google.com>