run db-sync with OpenStack users

When db-sync is run by OpenStack CLI tools (i.e. nova-manage), it writes
in /var/log/ with root user.
In the case where we don't use syslog, the service can't start since it
will try to write as non-root user in the log file.

This patch aims to run the commands with OpenStack users.
Merge pull request #427 from enovance/fix-db-sync

run db-sync with OpenStack users