ec2_vpc_endpoint: make policy_path usable (#31801)

The current code flow precludes the use of the policy_path module
parameter that's documented.  It's actually called policy_file in the
code.

What's worse is that the policy_file branch actually tries to open the
file named by the policy parameter, even though policy and policy_file
are marked as mutually-exclusive.

This change fixes the logic bug in policy_file and updates the
documentation to reference policy_file.  The old parameter policy_path
is provided as an alias

lib/ansible/modules/cloud/amazon/ec2_vpc_endpoint.py

@@ -39,7 +39,7 @@ options:
       - Option when creating an endpoint. If not provided AWS will
         utilise a default policy which provides full access to the service.
     required: false
-  policy_path:
+  policy_file:
     description:
       - The path to the properly json formatted policy file, see
         U(https://github.com/ansible/ansible/issues/7005#issuecomment-42894813)
@@ -47,6 +47,7 @@ options:
       - Option when creating an endpoint. If not provided AWS will
         utilise a default policy which provides full access to the service.
     required: false
+    aliases: [ "policy_path" ]
   state:
     description:
         - present to ensure resource is created.
@@ -262,7 +263,7 @@ def create_vpc_endpoint(client, module):
 
     elif module.params.get('policy_file'):
         try:
-            with open(module.params.get('policy'), 'r') as json_data:
+            with open(module.params.get('policy_file'), 'r') as json_data:
                 policy = json.load(json_data)
         except Exception as e:
             module.fail_json(msg=str(e), exception=traceback.format_exc(),
@@ -330,7 +331,7 @@ def main():
             vpc_id=dict(),
             service=dict(),
             policy=dict(type='json'),
-            policy_file=dict(type='path'),
+            policy_file=dict(type='path', aliases=['policy_path']),
             state=dict(default='present', choices=['present', 'absent']),
             wait=dict(type='bool', default=False),
             wait_timeout=dict(type='int', default=320, required=False),