updates in bin/prepare_test_env.sh
- rename from prepare_test_highstate_env.sh to prepare_test_env.sh as it
is used for tests that don't do highstate at all
- use a new flag -g to install packages needed for highstate, and to do
various other preparations like the /etc/salt/grains
- restructure the secrets conditionals to make it a bit faster by
avoiding the grep if the -s flag is not set