restapi: add usages field to "add network to data-center" action params

1. Add network usages field to "add network" action parameters.
2. Copy optional parameters definition from "/networks|rel=add" URL to
"/datacenters/{datacenter:id}/networks|rel=add" URL.
3. Re-arrange the parameter blocks for better readability.

Change-Id: Ia5549de3adee242801cd6034426ec67310cbf8d6
Bug-Url: https://bugzilla.redhat.com/1085464
Signed-off-by: Yevgeny Zaspitsky <yzaspits@redhat.com>