prometheus: upgrade alertmanager to 0.15.0

- remove unused var

roles/openshift_prometheus/defaults/main.yaml

@@ -9,7 +9,7 @@ openshift_prometheus_namespace: openshift-metrics
 l_openshift_prometheus_version_dict:
   origin:
     prometheus: 'v2.2.1'
-    alert_manager: 'v0.14.0'
+    alert_manager: 'v0.15.0'
     alert_buffer: 'v0.0.2'
     node_exporter: 'v0.16.0'
   openshift-enterprise:
@@ -42,7 +42,6 @@ openshift_prometheus_alerts_hostname: alerts-{{openshift_prometheus_namespace}}.
 openshift_prometheus_alertmanager_hostname: alertmanager-{{openshift_prometheus_namespace}}.{{openshift_master_default_subdomain}}
 
 l_openshift_prometheus_image_version: "{{ openshift_prometheus_image_version | default('v2.2.1') }}"
-l_openshift_prometheus_alertmanager_image_version: "{{ openshift_prometheus_alertmanager_image_version | default('v0.14.0') }}"
 l_openshift_prometheus_alertbuffer_image_version: "{{ openshift_prometheus_alertbuffer_image_version | default('v0.0.2') }}"
 
 openshift_prometheus_node_selector: "{{ openshift_hosted_infra_selector | default('node-role.kubernetes.io/infra=true') | map_from_pairs }}"