Open Stack

We're experiencing an error (the root cause of https://bugs.launchpad.net/nova/+bug/1135465 ) with some tempest tests which need a devstack environment set up on two XenServer hosts - one running all services, and the second host as a compute node.
 
The main problem is that mysql can only be an ENABLED_SERVICE on one of the hosts - otherwise the mysql service on the "slave" devstack machine will wipe the database on the primary host.  This is because we're trying to use MYSQL_HOST to house the IP address for the primary devstack machine. 

Without mysql in an ENABLED_SERVICE, sql_connection in /etc/nova/nova.conf is never populated.  This is been compounded by https://review.openstack.org/#/c/22635/ which deprecates setting of DATABASE_TYPE directly, and forces mysql to be added to the ENABLED_SERVICES if DATABASE_TYPE is defined in the localrc.

What should we be setting to use devstack to set up a second compute node?

Thanks,

Bob