Making the loader.js using configurable service url

The `api-url` used by `build-ga.py`is defined in the environment related `buildout.cfg` files.

You need to test against the branch geoadmin/ol3/tree/fix-service_url  (see geoadmin/ol3/pull/62), because new externals are defined, and you need to merge geoadmin/ol3/pull/62 and `./buildout/bin/buildout install update ol3-update` before merging this PR: