Run daemons as appropriate user

In ops manual: when running a daemon from the CLI, run as the
appropriate user (e.g., glance, nova, keystone) instead of as root.

This will catch incorrect permission errors that wouldn't appear when
running as root.

Cherry-picked from https://review.openstack.org/25240

Change-Id: Iccc112cf69de5c9d2c96dce2dd27e804ae5f7b8e