restapi: remove display field from add network to data-center
action parameters

Remove display field from add network to data-center action parameters
as it isn't relevant to the action on data-center level.

Change-Id: Ie8a135347ccdd3e8692b12a639c4ee214384e5e4
Signed-off-by: Yevgeny Zaspitsky <yzaspits@redhat.com>