Instead of relying on the user of the interface to set the correct
hostname parameter when configuring database access request,
automatically determine the correct hostname using:
1) In Juju >= 2.0, using the network space binding of the
named relation.
2) In Juju < 2.0, using the units private-address.
The hostname relation value can be overridden using the hostname
parameter of the configure method.
Change-Id: Id69a3786d2ac754aa8659b8cde977c4d86a37fc8