Run glance-manage db_version as glance user to avoid creating /var/log/glance/registry.log as root

The not_if command doesn't seem to run as user glance, so we have to explicitly set it to avoid /var/log/glance/registry.log being created as root user (the db_version writes to /var/log/glance/registry.log).  This is currently happening on CentOS 6.x and is preventing the migrations from being run.