Netconf's yang-ext:mount nodes not registered for remote RPC handling

When a Netconf device is configured on one node of a cluster and then accessed on a different node, its state is visible but any attempt to access its yang-ext:mount URL will end up with "Mount point does not exist".

Testsuite exposing the problem is here: https://git.opendaylight.org/gerrit/#/c/28725

Occurs on stable/lithium and on beryllium as well. The symptoms and behavior exhibited when they are subjected to the test suite linked to above is exactly the same.