fix fieldset look, fix #8158

Before it looked like this (from issue #8158):
![owncloud bug 2](https://cloud.githubusercontent.com/assets/2016878/2674123/bc95b664-c0fb-11e3-8d4d-95df254da398.png)

Now it looks like this (header layout fixed):
![owncloud installation errors](https://cloud.githubusercontent.com/assets/925062/3486073/74a35dd0-041b-11e4-872f-8fd6d9f7af90.png)

Please review @Xenopathic @owncloud/designers