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>