Modify OpenRoadmInterfacesImpl221 implementation

When we create a new interface on a device, we need to just after update the mapping to take into account this new interface. Unfortunatelly, it may happen sometimes that the new interface is not present yet in the operational device datastore at the moment of the update mapping occurs.
To avoid such roll-back root cause error, it is necessary to introduce in the postInterface method a check of the presence of the new just created interface before returning.