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>
>..test engine try to read default target(s) from the relevant array of these two. (If the array doesn't exists then the engine uses the first cluster from Clusters array as default.)

..test engine uses the default target(s) from one of these default definitions. If undefined, then the engine uses the first cluster from the Cluster array)