More flexible environment variable overrides

Overriding configuration parameters with environment variables used to
work by walking the configuration structure and checking for a
corresponding environment variable for each item. This was very limiting
because only variables corresponding to items that already existed in
the configuration structure would be checked. For example, an
environment variable corresponding to nested maps would only be noticed
if the outer map's key already existed.

This commit changes environment variable overriding to iterate over the
environment instead. For environment variables beginning with the
REGISTRY_ prefix, it splits the rest of their names on "_", and
interprets that as a path to the variable to unmarshal into. Map keys
are created as necessary. If we encounter an empty interface partway
through following the path, it becomes an implicit
map[string]interface{}.

With the new unit tests added here, parser.go now has 89.2% test
coverage.

TestParseWithExtraneousEnvStorageParams was removed, because the limit
of one storage driver is no longer enforced while parsing environment
variables. Now, Storage.Type will panic if multiple drivers are
specified.

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

configuration/configuration.go

@@ -11,7 +11,10 @@ import (
 )
 
 // Configuration is a versioned registry configuration, intended to be provided by a yaml file, and
-// optionally modified by environment variables
+// optionally modified by environment variables.
+//
+// Note that yaml field names should never include _ characters, since this is the separator used
+// in environment variable names.
 type Configuration struct {
 	// Version is the version which defines the format of the rest of the configuration
 	Version Version `yaml:"version"`
@@ -305,6 +308,8 @@ type Storage map[string]Parameters
 
 // Type returns the storage driver type, such as filesystem or s3
 func (storage Storage) Type() string {
+	var storageType []string
+
 	// Return only key in this map
 	for k := range storage {
 		switch k {
@@ -317,9 +322,15 @@ func (storage Storage) Type() string {
 		case "redirect":
 			// allow configuration of redirect
 		default:
-			return k
+			storageType = append(storageType, k)
 		}
 	}
+	if len(storageType) > 1 {
+		panic("multiple storage drivers specified in configuration or environment: " + strings.Join(storageType, ", "))
+	}
+	if len(storageType) == 1 {
+		return storageType[0]
+	}
 	return ""
 }