fix(capacity_reservation): use it when adding nodes in runtime

seems like by mistake the code was trying to read SCT configuration
from the wrong place, cause capacity_reservation to be ignroe
when nodes are added during the test.

sdcm/cluster_aws.py

@@ -150,7 +150,7 @@ def _create_on_demand_instances(self, count, interfaces, ec2_user_data, dc_idx=0
         subnet_info = ec2.get_subnet_info(interfaces[0]["SubnetId"])
         region_name_with_az = subnet_info['AvailabilityZone']
         ec2.add_placement_group_name_param(params, self.placement_group_name)
-        if params.get('use_capacity_reservation'):
+        if self.params.get('use_capacity_reservation'):
             ec2.add_capacity_reservation_param(params, region_name_with_az[-1])
         LOGGER.debug('Sending an On-Demand request with params: %s', params)
         LOGGER.debug('Using EC2 service with DC-index: %s, (associated with region: %s)',