Automatic spanloss measurement

This is related to TRNSPRTPCE-95, where it is only mentioned that spanloss is needed in the OMS attribute in the network topology for OSNR calculation. But spanloss is also read from the ROADM device when calculating and setting the power in PowerMgmt.java. Currently, it seems that a spanloss measurement has to be triggered manually through RESTCONF API in order to write the spanloss to the device. Otherwise power setup will fail. So I think automatic spanloss measurement when a ROADM-to-ROADM link is discovered is needed even without OSNR calculation.