External locking for image caching.
External locking for image caching.

If the instance storage is shared between compute nodes, then you
need external locking which is also shared to avoid clobbering each
other's attempts to cache base images. Resolves bug 1014227.

Change-Id: Ic2ac87840904fa199c17774dae9556ad6c7a3eaf

openstack/oslo.versionedobjects,citrix-openstack-build/oslo.versionedobjects