Fix database connection section

Using the sql_connection option in the default section appears to
cause CONF.api_database.connection to be populated with the
connection uri. This in turn causes nova to think this is an
api cell and to do an api lookup *1 which fails. This change moves
the database connection string into its own section as per *2

*1 https://opendev.org/openstack/nova/src/branch/master/nova/utils.py#L1060
*2 https://docs.openstack.org/nova/latest/admin/cells.html

Closes-Bug: #1982071
Change-Id: I49859235d408598535173c8bb87ce65f66fb3ca8

src/templates/xena/nova.conf

@@ -0,0 +1,26 @@
+# train
+###############################################################################
+# [ WARNING ]
+# Configuration file maintained by Juju. Local changes may be overwritten.
+###############################################################################
+[DEFAULT]
+verbose={{ options.verbose }}
+debug={{ options.debug }}
+logdir=/var/log/nova
+state_path=/var/lib/nova
+root_helper=sudo nova-rootwrap /etc/nova/rootwrap.conf
+
+{% include "parts/section-transport-url" %}
+
+[conductor]
+workers = {{ options.workers }}
+
+{% include "parts/section-oslo-messaging-rabbit" %}
+
+{% include "parts/section-placement" %}
+
+[database]
+{% if shared_db.host -%}
+connection = {{ shared_db.uri }}
+{% endif -%}
+