Add attaching state for Volumes
Add attaching state for Volumes

bug 897726 Add "attaching" status for volumes and clear state on error
This closes a gap where multiple calls of attach for the same volume
can happen before the volume is actually attached.

Change-Id: I59d22ceda83729c1a455af9994c9ffec1912e23b

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