Fix invalid value for property api_endpoint_overrides/storage

[carlosmerec]

When the shim is used the following error is thrown by gcloud:

Invalid value for property [api_endpoint_overrides/storage]: The endpoint_overrides property must be an absolute URI beginning with http:// or https:// and ending with a trailing '/'. [https://storage.googleapis.com/storage/v1] is not a valid endpoint override.

This is in case the user passes the gs_json_host config variable.

[carlosmerec]

Kokoro tests: the linux ones fail due to BUILD FAILED and the mac os one is because of a test failure but it seems it is unrelated.