config validation: validator only checks for file presence not content

when file system is used to configure Envoy, the config validator (`-mode validate`) only checks if files are present but does not load those files to check if their content is valid.

In this super simple config:
```
dynamic_resources:
cds_config:
resource_api_version: V3
path_config_source:
path: /test/cds.yaml
lds_config:
resource_api_version: V3
path_config_source:
path: /test/lds.yaml
```

the validator will accept empty and non-yaml files when run in `validate` mode.