ipa host-add: do not raise exception when reverse record not added
    
    When ipa host-add --random is unable to add a reverse record (for instance
    because the server does not manage any reverse zone), the command
    adds the host but exits (return code=1) with an error without actually
    outputing the random password generated.
    With this fix, the behavior is modified. The commands succeeds (return code=0)
    but prints a warning.
    
    This commit also adds a unit test.
    
    https://pagure.io/freeipa/issue/7374
    
    Reviewed-By: Christian Heimes <cheimes@redhat.com>