add RESTCONF capability for schemaless mount

it would be good if we could use the RESTCONF (and NETCONF) NBs to access schemaless mounts. The current schemaless mount code doesn't allow this.

Given that there's no YANG model to infer the URL structure for RESTCONF I suspect the best way to handle this is to create new RPCs that get, edit and delete data for schemaless devices - and which take the device name, subtree or xpath, and (for edit) the anyxml data to write as parameters.

This was requested by Hu Wenbo <huwenbo1988@gmail.com>.