Fix Share Network Subnet Metadata policy error

When the user logged is not an admin, the project find for the
policies using some information like project_id. Since the
Subnet model doesn't have project_id, the new property refering
the Share Network project_id now is used to guarantee access

Closes-bug: #2011869
Change-Id: I58331fa6906c855804abb94fad85a8370625c886
(cherry picked from commit 1a60368)

manila/db/sqlalchemy/models.py

@@ -1066,6 +1066,10 @@ def subnet_metadata(self):
             metadata_dict[meta['key']] = meta['value']
         return metadata_dict
 
+    @property
+    def project_id(self):
+        return self.share_network['project_id']
+
 
 class ShareNetworkSubnetMetadata(BASE, ManilaBase):
     """Represents a metadata key/value pair for a subnet."""

releasenotes/notes/bug-2006792-fix-add-share-network-subnet-non-admin-463347a723069997.yaml

@@ -0,0 +1,6 @@
+---
+fixes:
+  - |
+    Share Network Subnet Metadata now can be add by any user, as the
+    project_id used to verify the policy in the case of a non-admin user
+    now is present in the Share Network Subnet db model.