validate plugin option

When the plugin is loaded for the first time we need to check that the options are correct, especially that the link user_schema_path is defined and the file is a json object.
If the schema has't the default properties email and password:
``
{
"email": {"type": "email"},
"password": {"type": "password"},
}

```
initialise the schema with these values and write back the file.
```