Ganeti does not support optional fields in parameters
(hypervisor-params, disk-params, etc.). OpenVSwitch related node
parameters were the exception to this rule, which caused numerous
problems related to import/export and (de-)serialization.

The reason for making those parameters optional in the first place was to
disallow them when OpenVSwitch is not used. This was not consistent with
other parts of Ganeti, where we allow parameters to be set even though they
are not actively used.

This patch makes all node parameters mandatory and provides sensible
defaults for them. Checks which make sure that certain parameters are
not set in some cases were removed, and the tests adapted. Also, the
inheritance logic from cluster -> node group -> node was implemented, as
it was missing previously.

Signed-off-by: Thomas Thrainer <thomasth@google.com>
Reviewed-by: Hrvoje Ribicic <riba@google.com>