Change to oslo.db requires changes to conf

The following change to oslo.db requires a change in designate.conf.
database_connection in the conf file is now connection.
https://github.com/stackforge/designate/commit/9d5d70e7f16d4ec344d5663a0
e38d278cd543ef3#diff-f3c262a7ffc642899e6ff5b4fa7b3588
OR
https://review.openstack.org/#/c/98122/