[JUJU-5140] Write controller configuration file

[manadart]

Introduction of the configuration file as per specification

This patch ensures:

• that upon installation, the configuration file exists at the specified path and;
• when the dbcluster relation data for the application has a changed value for db-bind-addresses, the new value is written to the configuration file.

To verify:

• charmcraft pack this charm.
• Bootstrap a new LXD controller using --controller-charm-path <path-to-packed-artefact>.
• juju enable-ha.
• Once all units have joined the relation, ssh to the controller machine and check /var/lib/juju/agents/controller-0/agent.conf. It should be populated along the lines of:

db-bind-addresses:
  controller/0: 10.246.27.181
  controller/1: 10.246.27.53
  controller/2: 10.246.27.12

[SimonRichardson]

I think we need a unit test to ensure the file is correctly written.