Update start-flower in flower to wait until all celery workers are online

[arnav13081994]

Flower needs to start only after the celery workers come online. otherwise one will see errors like inspect method failed etc

Description

Checklist:

• [ ] I've made sure that tests are updated accordingly (especially if adding or updating a template option)
• [ ] I've updated the documentation or confirm that my change doesn't require any updates

Rationale