HPCC-11071 Regression setup needs to run on multiple clusters

Add 'setup' positional parameter to Regression Suite command line interface.

Change target parameter name from 'cluster' to 'target'

Remove setup from cluster list.

Remove setup as a valid target for run command.

Redesign main script structure:
    - move whole code into a class (to easier unit testing in future)
    - move long IF-ELSIF-..-ELSE code blocks into individual methods
    - add missing target testing for setup and run commands

Change password to '********' string in debug log.

Update README.rst

Signed-off-by: Attila Vamos <attila.vamos@gmail.com>
Merge pull request #5674 from AttilaVamos/HPCC-11071-improvement

HPCC-11071 Regression setup needs to run on multiple clusters

Reviewed-by: Gavin Halliday <ghalliday@hpccsystems.com>