tests: cover slapos usage
Add some doctests to describe the use cases of shared parts with slapos, including the need for our `${:_profile_base_location_}` buildout patch and the need to save signature files.
Add some doctests to describe the use cases of shared parts with slapos, including the need for our `${:_profile_base_location_}` buildout patch and the need to save signature files.