Surface custom handlers with custom environments

[adriangonz]

When using custom environments, the model can't be even imported within the main process. Therefore, the custom handlers hook is not able to inspect the model to look for custom handlers which need to registered within the REST server.

Because of this reason, for models not using any custom environment (i.e. where model_settings.parameters.environment_tarball is empty), we revert back to the default model initialiser (patched in #1104 ).