manager: add infrastructure for controlling systemd services

[bmr-cymru]

Add a new private module, snapm.manager._services to integrate with systemd and provide primitives for controlling services.

Ref: The D-Bus API of systemd/PID 1

This should be modelled in the existing low-level systemd timer interface in snapm.manager._timers and provide primitives for:

• Enumerating running services
• Stopping a running service
• Starting a stopped service
• Validating that an expected set of services is running