Add http.host parameter

This allows the administrator to specify an externally-reachable URL for
the registry. It takes precedence over the X-Forwarded-Proto and
X-Forwarded-Host headers, and the hostname in the request.

Signed-off-by: Aaron Lehmann <aaron.lehmann@docker.com>

configuration/configuration_test.go

@@ -65,6 +65,7 @@ var configStruct = Configuration{
 	HTTP: struct {
 		Addr   string `yaml:"addr,omitempty"`
 		Net    string `yaml:"net,omitempty"`
+		Host   string `yaml:"host,omitempty"`
 		Prefix string `yaml:"prefix,omitempty"`
 		Secret string `yaml:"secret,omitempty"`
 		TLS    struct {