Allow single character repository names

The main goal of this changeset is to allow repository name components to
consist of a single character. The number of components allowed and the slash
separation requirements have also been clarified.

To go along with this simplification, errant constants and unneeded error types
have been removed.

Signed-off-by: Stephen J Day <stephen.day@docker.com>

registry/api/v2/routes_test.go

@@ -263,6 +263,7 @@ func checkTestRouter(t *testing.T, testCases []routeTestCase, prefix string, dee
 		}
 
 		if testcase.StatusCode != http.StatusOK {
+			resp.Body.Close()
 			// We don't care about json response.
 			continue
 		}
@@ -291,6 +292,8 @@ func checkTestRouter(t *testing.T, testCases []routeTestCase, prefix string, dee
 		if deeplyEqual && !reflect.DeepEqual(actualRouteInfo, testcase) {
 			t.Fatalf("actual does not equal expected: %#v != %#v", actualRouteInfo, testcase)
 		}
+
+		resp.Body.Close()
 	}
 
 }