Adding Read-Only volume attaching support to Nova

Supporting attach a volume with Read-Only mode in libvirt driver.

Implements bp: read-only-volumes

Related-Id: I4c84614d6541d5f7c358abadb957da7b8c3d9c48
Change-Id: Id5ef7b25350cb1bde9d30fbfa98fb7a80081827f
Signed-off-by: Zhi Yan Liu <lzy.dev@gmail.com>
Merge "Adding Read-Only volume attaching support to Nova"
xenapi: post install scriptlet to rpm

A freshly installed XenServer requires some modifications in order to be
able to run OpenStack. Nova contains an rpm specification to create a
package with the dom0 plugins, however installing the rpm is not enough.
Some more actions need to be taken:
 - create /images, link to a directory within the local SR
 - setup an ssh keypair, and authenticate it with the host
This change adds a post script to the nova plugins rpm spec file, so
by installing the rpm on a XenServer, it is ready to run OpenStack, no
further manual actions needed.

Change-Id: Id57e3a3ab2782550468c6aa4c01059ceb7cc81ab