Fix validation error for ImagePullPolicy in charms

Change-Id: I00711202fdbacd73e9205dd3fc34424893440be5
Signed-off-by: sousaedu <eduardo.sousa@canonical.com>
diff --git a/installers/charm/keystone/src/charm.py b/installers/charm/keystone/src/charm.py
index 4e04e88..51ee6ad 100755
--- a/installers/charm/keystone/src/charm.py
+++ b/installers/charm/keystone/src/charm.py
@@ -84,7 +84,7 @@
     mysql_host: Optional[str]
     mysql_port: Optional[int]
     mysql_root_password: Optional[str]
-    image_pull_policy: Optional[str]
+    image_pull_policy: str
 
     @validator("max_file_size")
     def validate_max_file_size(cls, v):