Fix log rotate permission error

As described in #7651, logrotate produces errors with the current logrotate config shipped with gitlab.
This error showed up on my debian wheezy machine. Although I'm not sure I anyone else has encountert the same problem I found a solution for this problem:

Logrotate needs to know which permissions (user/group) it has to use when rotationg log files. The su directive has to be used to set correct permissions. Man page of logrotate:

su user group
        Rotate  log files set under this user and group
        instead of using default user/group (usually root).
        user specifies the user name used for rotation and group
        specifies the group used for rotation.

Please consider for pulling into master.