Adding Read-Only volume attaching support to Cinder client

Adding mode argument to volume manager attach function.
Allow Nova using read-write or read-only mode to attach a volume to an
instance.

blueprint read-only-volumes

Change-Id: Ib20c7802304d83f4aafef522171ebd47f672d883
Signed-off-by: Zhi Yan Liu <zhiyanl@cn.ibm.com>