Subtree filter does not return any data

From CSIT point of view, this Boron bug is the same as NETCONF-193 but this hopefully has a better description.

"Netconf readiness" suite started failing, but without any accompanying failures in subsequent functional tests.
Manual investigation showed that while accessing the whole modules container on controller-config works:

$ curl -v -u'admin:admin' 127.0.0.1:8181/restconf/config/network-topology:network-topology/topology/topology-netconf/node/controller-config/yang-ext:mount/config:modules | less

Attempt to look deeper (for data visible in the previous output) returns 404:

$ curl -v -u'admin:admin' 127.0.0.1:8181/restconf/config/network-topology:network-topology/topology/topology-netconf/node/controller-config/yang-ext:mount/config:modules/module/odl-sal-netconf-connector-cfg:sal-netconf-connector/controller-config ;echo

The date the test started to fail coincides with merge of https://git.opendaylight.org/gerrit/37486