To ensure that the charm presents the right IP address for
accessing the MySQL database over the shared-db relation,
ensure that any network space binding provided by the user
is preferred over the default of 'private-address'.

If network spaces is not supported (juju < 2.0), fallback to
using 'private-address'.

Change-Id: Ic2ed5d2cb88084c8eab33326bc16eb700ae9a35f