MXS-1354: Make new users backwards compatible

The `add user` and `enable account` commands create fully privileged
administrative users like they did in 2.1. This makes the addition of
read-only users backwards compatible.

Updated and expanded the documentation on administrative interface
users. Added entries into the release notes as well as the upgrading
document about relevant changes between 2.1 and 2.2.

Documentation/Upgrading/Upgrading-To-MaxScale-2.2.md

@@ -12,6 +12,12 @@ For a complete list of changes in MaxScale 2.2.0, refer to the
 Before starting the upgrade, we recommend you back up your current configuration
 file.
 
+### Administrative Users
+
+The file format for the administrative users used by MaxScale has been
+changed. Old style files are automatically upgraded and a backup of the old file is
+stored in `/var/lib/maxscale/passwd.backup`.
+
 ### Regular Expression Parameters
 
 Modules may now use a built-in regular expression string parameter type instead