Add cinder-manage cmd to update host column

Add cinder-manage cmd to update host column

If you deploy Cinder as per the default using the docs,
you can't later decide to add backends using the
multi-backend functionality. The reason is that multi-backend
is implemented by appending a backend-name to the host entry.

So if you convert the config to multi-backend the hostname
for any volumes that you have created are no longer
associated with a valid volume-driver (ie you can no
longer perform operations including delete on these
volumes).

In addition, in the case of using a SAN attached backend if
one redeploys their cinder-volume service to another node
they'll have the same issue.

This patch adds a cinder-manage command that will go through
volumes that are located on a specified host and update
the host column on each of them.

Change-Id: I8758fb8ef3fccbba2a598240c42d625b997e2db6
Closes-Bug: 1320688