Run bundle as the config user instead of hardcoding to 'deploy'

Currently **bundle** is being run as the **deploy** user while the install path for bundler is set to the configured home directory. If we use a custom user for deployment, then the home directory will be the custom user's home directory, but the hard-coded *deploy* user won't have permissions to copy anything there & the bundle will fail. Hence we need to use the custom deployment user while running *bundle* - if no custom user is specified, this defaults to the *deploy* user anyway.