Removed extra work in case of no volumes

Before the change the following actions were performed even if there
are no volumes to attach:
1. thread spawn
2. roundtrip to instance to make sure that there are 0 volumes attached
3. roundtrip to nova to get assigned volumes

Change-Id: I3f120cb5e60b2c8b2439c7f4acc7b99ce4964895
Merge "Removed extra work in case of no volumes"