upgrade prometheus 2.1.0 -> 2.2.1

roles/openshift_prometheus/vars/default_images.yml

@@ -7,7 +7,7 @@ l_openshift_prometheus_alertbuffer_image_prefix: "{{ openshift_prometheus_alertb
 l_openshift_prometheus_node_exporter_image_prefix: "{{ openshift_prometheus_node_exporter_image_prefix | default(l_openshift_prometheus_image_prefix) }}"
 
 # image version defaults
-l_openshift_prometheus_image_version: "{{ openshift_prometheus_image_version | default('v2.1.0') }}"
+l_openshift_prometheus_image_version: "{{ openshift_prometheus_image_version | default('v2.2.1') }}"
 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('v0.14.0') }}"
 l_openshift_prometheus_alertbuffer_image_version: "{{ openshift_prometheus_alertbuffer_image_version | default('v0.0.2') }}"