Refactor a little so 'ros os list' also uses the configured proxy info

Signed-off-by: Sven Dowideit <SvenDowideit@home.org.au>

config/cloudinit/datasource/url/url.go

@@ -19,6 +19,7 @@ import (
 
 	"github.com/rancher/os/config/cloudinit/datasource"
 	"github.com/rancher/os/config/cloudinit/pkg"
+	"github.com/rancher/os/util/network"
 )
 
 type RemoteFile struct {
@@ -31,6 +32,7 @@ func NewDatasource(url string) *RemoteFile {
 }
 
 func (f *RemoteFile) IsAvailable() bool {
+	network.SetProxyEnvironmentVariables()
 	client := pkg.NewHTTPClient()
 	_, f.lastError = client.Get(f.url)
 	return (f.lastError == nil)