Fix validation error for ImagePullPolicy in charms

Change-Id: I00711202fdbacd73e9205dd3fc34424893440be5
Signed-off-by: sousaedu <eduardo.sousa@canonical.com>
diff --git a/installers/charm/mon/src/charm.py b/installers/charm/mon/src/charm.py
index f253c09..e134041 100755
--- a/installers/charm/mon/src/charm.py
+++ b/installers/charm/mon/src/charm.py
@@ -80,7 +80,7 @@
     grafana_user: str
     grafana_password: str
     certificates: Optional[str]
-    image_pull_policy: Optional[str]
+    image_pull_policy: str
 
     @validator("log_level")
     def validate_log_level(cls, v):