Move service user injection into MySQL authenticator

The MySQL authenticator now injects the service user into the list of
allowed users if loading of database users fails. This allows the removal
of common code in the binlogrouter and maxinfo modules.

Documentation/Authenticators/MySQL-Authenticator.md

@@ -32,3 +32,21 @@ port=4006
 authenticator=MySQLAuth
 authenticator_options=cache_dir=/tmp
 ```
+
+### `inject_service_user`
+
+Inject service credentials into the list of database users if loading of
+users fails. This option takes a boolean value and it is enabled by
+default.
+
+When a connection to the backend database cannot be made, the service user
+can be injected into the list of allowed users. This allows administrative
+operations to be done via the SQL interface with modules that support it
+e.g. the Binlogrouter and Maxinfo modules.
+
+If users are loaded successfully, the service user credentials are _not_
+injected into the list of users.
+
+```
+authenticator_options=inject_service_user=false
+```