Adding ImagePullPolicy config option to OSM Charms

Change-Id: I04ad8444088e8a360755dc4e22b7ea53942682b2
Signed-off-by: sousaedu <eduardo.sousa@canonical.com>
diff --git a/installers/charm/prometheus/config.yaml b/installers/charm/prometheus/config.yaml
index c1c0fb9..6ce1613 100644
--- a/installers/charm/prometheus/config.yaml
+++ b/installers/charm/prometheus/config.yaml
@@ -65,3 +65,9 @@
     type: boolean
     description: Boolean to enable the web admin api
     default: false
+  image_pull_policy:
+    type: string
+    description: |
+      ImagePullPolicy configuration for the pod.
+      Possible values: always, ifnotpresent, never
+    default: always