Run cinder-manage as correct user/group

We need to explicitly run cinder-manage with cinder user/group.
Running as root may cause files in /var/log/cinder/* to be owned by
root, and this will cause cinder services from being able to start (due
to being unable to write to files in that directory).

This is a non-issue when you install from packages as the package
post-install typically does a cinder-manage with the correct user, and
this ensures logs are created w/ correct ownership.

Change-Id: I50c94646a993ae5eb2e8775d979a4c0a73f2f4f2