[WIP] Add provisioning_api app

Issue to track the addition of the provisioning_api to the core.

Fixes self.

Todo:
 - [ ] Check unit tests
 - [ ] Check merge situations (with existing core routes)
- [ ] key has to be added in here https://github.com/owncloud/documentation/blob/master/go.php#L12
- [ ] please follow naming convention 'admin-provisioning-api'