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>
>run the cluster(s) suite. If target = 'all' then run suite on all clusters. If not defined then default value(s) come from config (ecl-test.json by default).

would read better as
```
run the suite on target cluster(s). If target = 'all' then run suite on all clusters. If undefined the config 'defaultTargetClusters' value will be used.
```