Mitaka requires the use of a separate nova-api database;
ensure that this is setup on the shared-db relation and add
a new [api_database] section to the mitaka nova.conf file.
Add a new NovaAPISharedDBContext wrapper to prefix the key
names for the nova-api database with 'nova_api_' so that we
can present two database connections to the same configuration
file.
Ensure that 'nova-manage api_db sync' is called for database
setup for >= mitaka release.
For upgrades, ensure that the database migration and service
start is deferred until the nova_api database has been
setup by the related mysql service.
Closes-Bug: #1556896
Change-Id: If1c9a037b1e0745a486a57fddf885e26aa7e313d