Fix typeerror from plugin_tests.py

- In our unittest we defined 'is_installed' to be a bool when in fact we wanted
  to verify if a package is installed via the _method_ is is_installed. Since
  overriding that method with our variable definition we basically removed
  any functionality of 'is_installed' method in Plugin class.

Signed-off-by: Adam Stokes <adam.stokes@ubuntu.com>
More pep style adjustments, now all unittest should pass

Signed-off-by: Adam Stokes <adam.stokes@ubuntu.com>