Only allow permission modification by the owner of a unit

[pLeminoq]

Currently write permissions are needed to change/register/remove a unit config. This is problematic because having write permissions effectively grants every other permission by just changing the unit config. Therefore components like the cloud connector, that need write permissions to register new units,
would also posses every other permission.
The solution is, as in unix, to only allow permission modifications by the owner of the unit. If no owner is defined the owner of the location of the unit should be taken.