bin/prepare_test_show_highstate_env.sh: Various improvements and bugfixes:
    
    - Use pillar/id/$hostname_$domain.sls instead of just hostname (the
      docker CI worker has ID which is just the hostname, but in our
      workstations it might not be the same)
    - do not write custom grains in pillar/id/$fqdn, but instead write them
      all in /etc/salt/grains, otherwise they are ignored by show_highstate
    - strip out the inclusion of the secrets files, as the docker CI runner
      is not able to read them