prevent the same logger from being configured twice

Forge sometimes gets itself into a situation where the same logger is defined twice in Kea. It would be nice if we could avoid that in the majority of tests.

For example, if you run test_radius[v6-radius-reservation-outside-pool-network-memfile], it will run:

• setup_server_with_radius(**configs[config_type])
• srv_control.build_and_send_config_files()

Both those calls add loggers, and the general logger gets added each time, for a total of two.

$ diff tests_results/test_radius_v6-radius-reservation-outside-pool-network-memfile_/kea-dhcp6.conf*
         "loggers": [
             {
                 "name": "kea-dhcp6",
                 "output_options": [
                     {
                         "output": "/opt/kea/var/log/kea.log",
                         "flush": true,
                         "maxsize": 10240000,
                         "maxver": 1,
                         "pattern": ""
                     }
                 ],
                 "debuglevel": 99,
                 "severity": "DEBUG"
+            },
+            {
+                "name": "kea-dhcp6",
+                "output_options": [
+                    {
+                        "output": "/opt/kea/var/log/kea.log"
+                    }
+                ],
+                "severity": "DEBUG",
+                "debuglevel": 99
             }
         ],

I think we don't want this behavior by default. If anything, testing two loggers with the same name should be an individual separate test.