Handle old-agents that don't support list_disk rpc call
Handle old-agents that don't support list_disk rpc call

We fall back to the disk info stored in the DB.

Change-Id: I8bff9f7b9e7677568f4c8b24df7a8cc8d2ddfd0e

barthy1/bosh,barthy1/bosh,barthy1/bosh,barthy1/bosh