Honour Java system proxy settings

[mithomas]

Currently the gradle plugin does no support the default Java proxy settings http(s).proxyHost/http(s).proxyPort/http(s).nonProxyHosts.

Even though it is possible to pass through the settings like so:

artifactory {
  clientConfig.proxy.host = System.properties['http.proxyHost']
  clientConfig.proxy.port = System.properties['http.proxyPort'] as Integer
}

this falls short for non-proxy-hosts or for different http/https proxies.