Fix S3 proxy property configuration

Since NXP-17358 the lookup of the proxy properties for S3 is done incorrectly

What's expected are the global:

• nuxeo.http.proxy.host
• nuxeo.http.proxy.port

But the system properties effectively (and incorrectly) looked up are:

• nuxeo.s3storage.nuxeo.http.proxy.host
• nuxeo.s3storage.nuxeo.http.proxy.port