Multiple Thors defined on the same machines could cause cores at startup

Regression caused by changes to HPCC-25815. 

in the code that creates hostgroups to match the existing groups, no hostgroup is created if it matches one that already exists.

If a thor group is processed a derived _mirror hostGroup is created.  The wrong base name was being filled in for this mirror group if it matched one that already existed.

That then causes an internal error when the hostGroup ips are expanded out in subsequent processing - because the host group could not be found.

Follow on core is unrelated - probably due to objects being destroyed in the wrong order.