Update index.js

Adding this.entityInstance for router, service and controller template generation.
In fact managing a big project with a lot of generated entities having the same js files name isn't very practical moreover in the IDE all tabs will have the same label.

I've planned to use JHipster for a huge project. In this project there are currently ~150 model files. It would be nice to have differents names for each controller files.

Let me know