mgmt, lib: implement REPLACE operation

mgmt, lib: implement REPLACE operation

Replace operation removes the current data node configuration and sets
the provided value. As current northbound code works only with one
xpath at a time, the operation only makes sense to clear the config of
a container without deleting it itself. However, the next step is to
allow passing JSON-encoded complex values to northbound operations which
will make replace operation much more useful.

Signed-off-by: Igor Ryzhov <iryzhov@nfware.com>