Missing check for availability of linkmgmt.php

Testing with racktables 0.20.7:

I don't use the linkmgmt.php plugin, so I got a MySQL error when trying to copy objects, due to calling ```amplifyCell_object_Backend_Port()```. I simply copied the check for function ```linkmgmt_linkPorts``` that was also used in ```copyLotOfObjects()```, the error went away and the plugin works perfectly :)