Harden basic auth implementation

After consideration, the basic authentication implementation has been
simplified to only support bcrypt entries in an htpasswd file. This greatly
increases the security of the implementation by reducing the possibility of
timing attacks and other problems trying to detect the password hash type.

Also, the htpasswd file is only parsed at startup, ensuring that the file can
be edited and not effect ongoing requests. Newly added passwords take effect on
restart. Subsequently, password hash entries are now stored in a map.

Test cases have been modified accordingly.

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

docs/handlers/app.go

@@ -147,6 +147,7 @@ func NewApp(ctx context.Context, configuration configuration.Configuration) *App
 			panic(fmt.Sprintf("unable to configure authorization (%s): %v", authType, err))
 		}
 		app.accessController = accessController
+		ctxu.GetLogger(app).Debugf("configured %q access controller", authType)
 	}
 
 	return app