HPCC-9596 Core in dali if environment contains  nodes without @computer set

An environment that contains server nodes (Roxie or Thor) that do not have
@computer specified will core.

ConfigMgr should never create such an environment.

Signed-off-by: Richard Chapman <rchapman@hpccsystems.com>
Merge pull request #4577 from richardkchapman/dali-core-missing-computer

HPCC-9596 Core in dali if environment contains  nodes without @computer ...

Reviewed-by: Gavin Halliday <ghalliday@hpccsystems.com>