Removing first instance breaks WP access

[Jill Rouleau]

If site_url is not manually updated by the user after deployment the database will retain the IP of the first instance of wordpress. If another wordpress instance is deployed, then the original instance removed, site_url will still reflect the IP of the first instance in mysql.

To reproduce:

$ juju deploy wordpress
$ juju deploy mysql
$ juju add-relation wordpress mysql

Get the IP of wordpress/0, ex: 10.0.3.110

Access http://10.0.3.110 and set up wordpress.

$ juju deploy wordpress

IP of wordpress/1 is 10.0.3.21
Confirm can access wordpress site at 10.0.3.21

$ juju remove-unit wordpress/0

Access 10.0.3.21/wp-login.php
Logging in will redirect to 10.0.3.110/wp-login.php