mysql setup: if dbuser exists try a different one (owncloud/core#108)

Fixing the oldest bug available :)
This seams to be a proper solution for owncloud/core#108.

If dbuser has no admin rights, use it.
If dbuser has admin rights and oc_[admin] does not exist, create oc_[admin].
If dbuser has admin rights and oc_[admin] exists try oc_[admin]1, oc_[admin]2, ...

What do you think?
Thanks!