implement optional ControllerPublishVolume #80
Description
The csi-driver-host-path deployment with external-attacher is used to test that external-attacher works. However, because currently csi-driver-host-path doesn't actually implement ControllerPublishVolume, external-attacher uses a simplified code path (#78 (comment)).
To test the actual code path for drivers that support attach, csi-driver-host-path should get a boolean command line flag -controllerPublishVolume that, if set, enables an implementation of ControllerPublishVolume. That implementation doesn't need to do anything besides ensuring that ControllerPublishVolume and ControllerUnpublishVolume are called correctly. The node operations must check that ControllerPublishVolume was called. This can be done by creating some publish_context that is getting checked.