Adding host attaching support to Cinder

Changing 'os-attach' API interface to allow client mark a volume as be
attached to a host.

Implement bp: volume-host-attaching
docimpact

Change-Id: Iaf442ad0fb37ce369d838f3a512724f830071763
Signed-off-by: Zhi Yan Liu <zhiyanl@cn.ibm.com>
Merge "Adding host attaching support to Cinder"