Catalog for V2 API Implementation

This change adds a basic catalog endpoint to the API, which returns a list,
or partial list, of all of the repositories contained in the registry.  Calls
to this endpoint are somewhat expensive, as every call requires walking a
large part of the registry.

Instead, to maintain a list of repositories, you would first call the catalog
endpoint to get an initial list, and then use the events API to maintain
any future repositories.

Signed-off-by: Patrick Devine <patrick.devine@docker.com>

docs/api/v2/routes.go

@@ -16,6 +16,7 @@ const (
 
 var allEndpoints = []string{
 	RouteNameManifest,
+	RouteNameCatalog,
 	RouteNameTags,
 	RouteNameBlob,
 	RouteNameBlobUpload,

docs/api/v2/urls.go

@@ -100,6 +100,18 @@ func (ub *URLBuilder) BuildBaseURL() (string, error) {
 	return baseURL.String(), nil
 }
 
+// BuildCatalogURL constructs a url get a catalog of repositories
+func (ub *URLBuilder) BuildCatalogURL(values ...url.Values) (string, error) {
+	route := ub.cloneRoute(RouteNameCatalog)
+
+	catalogURL, err := route.URL()
+	if err != nil {
+		return "", err
+	}
+
+	return appendValuesURL(catalogURL, values...).String(), nil
+}
+
 // BuildTagsURL constructs a url to list the tags in the named repository.
 func (ub *URLBuilder) BuildTagsURL(name string) (string, error) {
 	route := ub.cloneRoute(RouteNameTags)