rebased from master.
If user faces locking related problem when two nova-compute hosts
sharing same disk area via nfs, try to download same image into
cache concurrently - Then base_dir_name can be set to "_base_$my_ip" in
nova.conf
Default value for base_dir_name is "_base" thus retaining existing
behavior.
Change-Id: Icff10ed75ba83f7256731614dc9e01e578b347a4