Add a default handler for storage-backend relation. This is part of the work to push cinder plugin bolierplate code into supporting layers. Change-Id: Icad5f72939b1e33d15adf2638f8e344235a9318b
Add a default handler for storage-backend relation. This is part of the work to push cinder plugin bolierplate code into supporting layers. Change-Id: Icad5f72939b1e33d15adf2638f8e344235a9318b