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