Add a identify-defaults options for import

When importing an instance, all the saved valued will be used as
explicitly specified values, overriding the cluster defaults. This means
export+import will change the status (from default to explicitly
specified) of parameters.

This patch adds a new option that changes the behaviour to identify
parameter values which are equal to the current cluster defaults and
mark them as such. It does this for hv, be and nic parameters.

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Michael Hanselmann <hansmi@google.com>