Adding Read-Only volume attaching support to Cinder

Adding Read-Only volume attaching support to Cinder

1. Adding an API extension to allow clients set volume Read-Only flag on
demand.
2. Require client to provide and be aware of volume attaching mode when
they call 'os-attach' API.
3. Adding a 'access_mode' field to connection info which
'os-initialize_connection' API returned. This field should be used by
client such as Nova to use correct mode accessing attached volume.
Currently access mode can be 'rw' or 'ro'.
4. In future, the driver within Cinder need to ensure the volume be
exposed under the correct access mode which connection info described,
for example backend should set volume to readonly mode when connection
info ask client using 'ro' access mode consume attached volume. That
means Read-Only is not only a attaching mode but also a status for a
volume.

blueprint read-only-volumes

Change-Id: I4c84614d6541d5f7c358abadb957da7b8c3d9c48
Signed-off-by: Zhi Yan Liu <zhiyanl@cn.ibm.com>