go/oasis-node/cmd: Allow joint validator and observer mode

This makes it consistent with the fact that node can currently
be validator and compute at the same time.

Author: martintomazic

go/oasis-node/cmd/node/node.go

@@ -489,7 +489,7 @@ func NewNode() (node *Node, err error) { // nolint: gocyclo
 		return nil, fmt.Errorf("node has no entity configured but expects one")
 	}
 
-	if config.GlobalConfig.Mode == config.ModeClient && entityID != nil {
+	if config.GlobalConfig.Mode == config.ModeClient && !config.GlobalConfig.Consensus.Validator && entityID != nil {
 		return nil, fmt.Errorf("client node must not have entity configured, try using observer mode instead")
 	}