Fix broken --list-disabled option

Invoking wptrunner with the --list-disabled throws an execption
 ```NameError: global name 'manifest' is not defined```

Stack trace:
```
Traceback (most recent call last):
  File "~/Dev/wptrunner/bin/wptrunner", line 9, in <module>
    load_entry_point('wptrunner==1.4', 'console_scripts', 'wptrunner')()
  File "~/Dev/wptrunner/wptrunner/wptrunner/wptrunner.py", line 657, in main
    list_disabled(**kwargs)
  File "~/Dev/wptrunner/wptrunner/wptrunner/wptrunner.py", line 528, in list_disabled
    test_loader = TestLoader(tests_root, metadata_root, TestFilter(), run_info)
  File "~/Dev/wptrunner/wptrunner/wptrunner/wptrunner.py", line 378, in __init__
    self.manifest = self.load_manifest()
  File "~/Dev/wptrunner/wptrunner/wptrunner/wptrunner.py", line 391, in load_manifest
    return manifest.load(self.manifest_path)
NameError: global name 'manifest' is not defined
```