set use_nova_server_config_drive to true

This allows nova file injection via config_drive to work which is needed
when trove adds configuration files to newly created instances via
cloud-init.

Note: the code to set this attribute is being backported in our Icehouse packages, but upstream does not have it in Icehouse, so the patches to openstack-database can also not go upstream. This nuisance will go away in Juno, but in the meantime, I don't know of a better way to handle this.
Any reason to need an attribute for this? I'd expect we always want this to true, don't we?