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.
4 jobs for feat/test_slapos_usages
Status Job ID Name Coverage
  External
passed SlapOS.Eggs.UnitTest-Jerome.Python2

01:07:07

running SlapOS.Eggs.UnitTest-Jerome.Python3

34266:41:57

running SlapOS.Eggs.UnitTest-Master.Python2

34265:29:11

running SlapOS.Eggs.UnitTest-Master.Python3

34265:29:48