Fix yarn failure when upgrading microservices

When generating an app without client code, upgrade generator fails because yarn complains about no licence. Adding `"private": true` to `package.json` avoids this error and makes a lot of sense as there'sn o npm module to publish anyway.

See yarnpkg/yarn#3821

generators/app/templates/_skipClientApp.package.json

@@ -17,6 +17,7 @@
  limitations under the License.
 -%>
 {
+  "private": true,
   "devDependencies": {
     <%_ otherModules.forEach(function(module) { _%>
     "<%= module.name %>": "<%= module.version %>",