Bootstrap updates

This PR is about remove the Bootstrap class and make the keycloak configuration more consistent on UPS.

- src/main/webapp/config/keycloak.json: configuration file for the client side aka admin-ui
- src/main/webapp/WEB-INF/keycloak.json: configuration file for RESTful endpoints

I did some tests and checked the issues reported here https://github.com/aerogear/aerogear-unifiedpush-server/pull/319. But worth a double check before we merge it.