Expose needs to track the service hierarchy

As part of the provisioning agent, the expose functionality needs to
be able to watch the service hierarchy and corresponding nodes to
supporting expose as follows:

 * Upon change to the service membership, services are watched for
   changes to being exposed or not.

 * Upon being exposed, the set of service units for each exposed
   service in turn are watched.

 * Upon a service unit's change in its open port settings, change the
   corresponding machine.

This watching should result in the following, which can be readily
tested: each possible change to a service unit, including deployment
and destroying of services, and how it is exposed then calls an
appropriate method in the provisioning agent (eg something like
ProvisioningAgent.manage_service_unit_ports(service_name,
service_unit_name)). This method should be instrumented such that it
can observed for testing to ensure it is called appropriately at every
time in the corresponding branch.

This method will also be used in a subsequent feature branch to tie
together changes to how the service hiearchy is exposed with specific
management for each of the corresponding provisioned machines.