HPCC-12033 Default target environment for regression suite should be configurable

Add "defaultSetupClusters" and "defaultTargetClusters" arrays to ecl-test.json
config file

Implement code to process default cluster(s) from config if it is not specified
in command line

Implement code to process cluster list in provided with -t |--target command
line parameter

Update built-in help and README.rst

Signed-off-by: Attila.Vamos <attila.vamos@gmail.com>
There's a separate issue opened re. adding a n-way Thor to same environment, but we decided that instead of that approach, we're going instead to have an alternative environment xml specifically for the regression suite.
So the vanilla environment.xml won't have a "thor3"

Regardless, I think default target clusters should also probably be "all"