nova-manage uses the [database]/connection to determine the
database connection. This change updates the nova.conf
template to use the parts/section-database template,
switching away from using [DEFAULT]/sql_connection to use
[database]/connection instead.
[DEFAULT/sql_connection has been deprecated since at least
mitaka:
https://docs.openstack.org/nova/mitaka/sample_config.html
This change also re-enables the gate tests for Xena.
Closes-Bug: #1951874
Change-Id: I9d77b44ad3c40f3ee60fedc3f211f3ac085fcbb2
Corey Bryant