bug 1540799: openshift_prometheus: update alertmanager config file flag

Prometheus alertmanager as of 0.13.0 requires double dashes (posix) for
command line flags.  The double dashes should be backwards compat
with earlier versions of alertmanager.

roles/openshift_prometheus/templates/prometheus.j2

@@ -219,7 +219,7 @@ spec:
 
       - name: alertmanager
         args:
-        - -config.file=/etc/alertmanager/alertmanager.yml
+        - --config.file=/etc/alertmanager/alertmanager.yml
         image: "{{ l_openshift_prometheus_alertmanager_image_prefix }}prometheus-alertmanager:{{ l_openshift_prometheus_alertmanager_image_version }}"
         imagePullPolicy: IfNotPresent
         resources:

roles/openshift_prometheus/vars/default_images.yml

@@ -8,5 +8,5 @@ l_openshift_prometheus_alertbuffer_image_prefix: "{{ openshift_prometheus_alertb
 # image version defaults
 l_openshift_prometheus_image_version: "{{ openshift_prometheus_image_version | default('v2.0.0') }}"
 l_openshift_prometheus_proxy_image_version: "{{ openshift_prometheus_proxy_image_version | default('v1.0.0') }}"
-l_openshift_prometheus_alertmanager_image_version: "{{ openshift_prometheus_alertmanager_image_version | default('v0.9.1') }}"
+l_openshift_prometheus_alertmanager_image_version: "{{ openshift_prometheus_alertmanager_image_version | default('v0.13.0') }}"
 l_openshift_prometheus_alertbuffer_image_version: "{{ openshift_prometheus_alertbuffer_image_version | default('v0.0.2') }}"