Plugin reporting unbound variable when dumping config [show_config()].

./global_fn.sh: line 95: CERT_LEVEL: unbound variable

There's no impact on the plugin execution, but that function is used when troubleshooting issues on plugin logs.

NOTE: it's desired to backport this patch to v0.2

v0.2+

Always

1. Run the cert tool and collect the results
2. unpack the results archive
3. check the plugin.txt from any plugin instance: less results/podlogs/openshift-provider-certification/sonobuoy-10-openshift-kube-conformance-job-fabced07cc18451b/logs/plugin.txt  

#./runner.sh:17>  20230107-003959> [runner] Plugin Config: PLUGIN_NAME=[10-openshift-kube-conformance] PLUGIN_BLOCKED_BY=[05-openshift-cluster-upgrade] CERT_TEST_FILE=[]
#./runner.sh:17>  20230107-003959> [runner] Setup config done.
./global_fn.sh: line 95: CERT_LEVEL: unbound variable
#./global_fn.sh:12>  [signal handler] ERROR on line 98 ./runner.sh
#./runner.sh:17>  20230107-004000> [runner] check and wait for dependencies...

Dump of the configuration: https://github.com/redhat-openshift-ecosystem/provider-certification-plugins/blob/main/openshift-tests-provider-cert/plugin/global_fn.sh#L90-L104