rename_rsgroup overwriting the existing rsgroup.

rename_rsgroup overwriting the current rsgroup.

Steps:

1)add_rsgroup 'RSG1' and 'RSG2'

2)move_servers_rsgroup 'RSG1',['server1:port']

3)rename_rsgroup 'RSG1','RSG2'

 After performing step3 RSG1 overwriting to RSG2 and region servers added in RSG1 are not available now.

Ideally system should show error message Group already exists: RSG2