[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)
@tomneedham please update the xml like this https://github.com/owncloud/core/blob/master/apps/files_encryption/appinfo/info.xml#L13