support new property to enable aspera proxy

There's a problem the client ran into – in one scenario they use a proxy for some features but not for Aspera, in another scenario they do need to use a proxy for some features including Aspera.

Please update so that a non-blank nuxeo.http.proxy.host is not the sole determiner for using the proxy for the Aspera connection (a configuration property like nuxeo.aspera.use.proxy would satisfy the client's scenario)