workload status does not survive juju upgrade.

[Christopher Lee]

After setting up mediawiki and relating it to a database the status workload states all is good.
If one updates juju the status reverts back to "Blocked, Database required" even though the DB connection is still there and good.

This is due to config-changed hook checking for db-connected canary file, but is using a relative path.

I've fixed this by ensuring the file is referenced by an absolute path every time it's used.

To reproduce:

Bootstrap a controller, deploy mediawiki and mysql, relate them
 - juju bootstrap lxd/localhost mtest
 - juju deploy cs:mediawiki
 - juju deploy mysql
 - juju status
    > Note that mediawiki workload is blocked, "Database required."
 - juju relate mediawiki:db mysql:db
 - juju status
    > Note that mediawiki workload is active, "Ready"
 - Go to http:// page for mediawiki
    > Note that it's happy

 - juju upgrade-juju -m controller
 - juju status
    > Note that mediawiki workload is blocked, "Database required."
 - Go to http:// page for mediawiki
    > Note that it's happy (even though status says otherwise)