Add DBus interface to drive update-manager

[Alex Launi]

Hardware cert/validation lab would like to be able to drive update-manager via dbus before running tests. The reason for this vs. just using apt is to better test the desktop stack.

Would like the following to be available:
  - Check for updates
    - Were there any errors
  - Install updates
    - Were there any errors

The true/false nature of errors are more important than the actual errors themselves.