The percentage of users in the given track to which the uploaded APK(s)
should be rolled out.
If you enter no value here, or 100%, the app will be rolled out to all users.
For more information on staged rollouts, see the Google Play documentation:
https://support.google.com/googleplay/android-developer/answer/3131213
This field supports substituting environment variables in the form
${SOME_VARIABLE} or $SOME_VARIABLE at build time.