mgmtd: validate candidate yang tree before creating a config diff

mgmtd: validate candidate yang tree before creating a config diff

The candidate yang tree should be validated before `nb_config_diff` is
called. `nb_config_diff` ignores all prohibited operations and can
provide an empty change list because of this. For example, if a user
deletes a mandatory node from the candidate datastore and tries to make
a commit, they'll receive the "No changes found to be committed!" error,
because such a change is ignored by `nb_config_diff`. Instead, mgmtd
should tell the user that their candidate datastore is not valid and
can't be commited.

Signed-off-by: Igor Ryzhov <iryzhov@nfware.com>