ipauser: Do not try to modify user when not changing password

If a playbook to ensure the existence of a user contains 'random:false'
and 'update_password: always' is executed twice, the second execution
will raise an exception due to "No modifications to perform", as there
is actually nothing to modify.

The fix for the issue is to remove 'random' if it is not set to true, as
setting it to 'false' would have no effect on the user object.

Related: https://issues.redhat.com/browse/RHEL-4934