Service Unit Agent presence should be observable

[Gustavo Niemeyer]

The ServiceUnitState API should enable monitoring the running state of a Service Unit Agent, so that if the Service Unit goes away for longer than a given amount of time, the Machine Agent can take the needed measures (as defined in bug #617556).

Here is a suggested API:

class ServiceUnitState:

    def has_agent(self):
        """Return boolean deferred informing whether an agent is connected."""

    def watch_agent(self):
        """Return two boolean deferreds informing whether an agent is connected, and whether a change happened."""

    def connect_agent(self):
        """Inform Ensemble that this is a service unit agent alive."""

Note that this API is the same proposed for the MachineAgentState, in bug #617046.