mgmt, lib: differentiate DELETE and REMOVE operations

mgmt, lib: differentiate DELETE and REMOVE operations

Currently, there's a single operation type which doesn't return error
if the object doesn't exists. To be compatible with NETCONF/RESTCONF,
we should support differentiate between DELETE (fails when object
doesn't exist) and REMOVE (doesn't fail if the object doesn't exist).

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