Fix #175: users doesn't default to all, instead no users are read
Fix #175: users doesn't default to all, instead no users are read

* This was introduced when we first "defaulted" the `--users` argument (a long time ago).
* The fix is to act as if *no* `--users` argument is `--users` by itself, because what the defaulting code really did was to default the argument to `all`.

adorton-adobe/user-sync.py,adobe-apiplatform/user-sync.py,adobe-apiplatform/user-sync.py,adorton-adobe/user-sync.py