ipauser: Make return value depending on users parameter

The way how randompasswords are returned by the ipauser module depends
so far on the number of users that are handled by the module.

This is unexpected if for example a json file is provided with the users
parameter. As it might be unknown how many users are in the json file,
this behaviour is unexpected. The return should not vary in this case.

This chamge makes the return simply depend on the use of the users
paramater. As soon as this parameter is used, the return will always be:

"user": { "<the user>": { "randompassword": "<the user random password>" } }

In the simply case with one user it will be still

"user": { "randompassword": "<the user random password>" }

Fixes: #1052 (ipauser should consitently return randompasswords when
              used with users)