Fixes error during alarm evaluation

Adds unit test

Change-Id: I2679210b7ce13f8ffd714183c220b617236003f8
Signed-off-by: Benjamin Diaz <bdiaz@whitestack.com>
diff --git a/osm_mon/evaluator/service.py b/osm_mon/evaluator/service.py
index 0868e32..3865b17 100644
--- a/osm_mon/evaluator/service.py
+++ b/osm_mon/evaluator/service.py
@@ -55,15 +55,17 @@
 
     def _get_metric_value(self,
                           nsr_id: str,
-                          vnf_member_index: int,
+                          vnf_member_index: str,
                           vdur_name: str,
                           metric_name: str):
-        return BACKENDS[self.conf.get('evaluator', 'backend')]().get_metric_value(metric_name, nsr_id, vdur_name,
-                                                                                  vnf_member_index)
+        return BACKENDS[self.conf.get('evaluator', 'backend')](self.conf).get_metric_value(metric_name,
+                                                                                           nsr_id,
+                                                                                           vdur_name,
+                                                                                           vnf_member_index)
 
     def _evaluate_metric(self,
                          nsr_id: str,
-                         vnf_member_index: int,
+                         vnf_member_index: str,
                          vdur_name: str,
                          metric_name: str,
                          alarm: Alarm):