Keep support for SPARK_PORTS, used in CentOS7 systemuser image

SPARK_PORTS is consumed by the port allocator version that runs
in the systemuser CentOS7 image. We need to keep compatibility
with such version for as long as we support the CentOS7 image.

swan-cern/files/swan_computing_config.py

@@ -365,10 +365,13 @@ async def _open_ports(self, num_ports):
             await self.spawner.api.replace_namespaced_service(computing_ports_service, swan_container_namespace, service)
 
             # Add ports env for computing integrations
+            # Keep old SPARK_PORTS variable name for as long as we support the CentOS7 image, since the port
+            # allocator version of such image expects a variable with that name
+            ports_var_name = 'COMPUTING_PORTS' if 'swan-cern' in notebook_container.image else 'SPARK_PORTS'
             notebook_container.env = self._add_or_replace_by_name(
                 notebook_container.env,
                 V1EnvVar(
-                    name='COMPUTING_PORTS',
+                    name=ports_var_name,
                     value=','.join(ports)
                 )
             )