az ml run submit-script does not load run config from source directory

- If the issue is to do with Azure CLI 2.0 in-particular, create an issue here at [Azure/azure-cli](https://github.com/Azure/azure-cli/issues)

### Extension name (the extension in question)

ml

### Description of issue (in as much detail as possible)

When running this command line:
```
az ml run submit-script --source-directory mydir -c myconf
```

the config is not loaded from mydir/aml_config/myconf

The cause is obvious in the extension source code: the path is used to load the run_config, instead of the source_directory argument

```
def submit_run(
experiment=None,
path=None,
run_configuration_name=None,
source_directory=None,
conda_dependencies=None,
run_async=None,
ct_name=None,
user_script_and_arguments=None,
logger=None):
...

if run_configuration_name is None:
logger.info("No Run Configuration name provided, using default: local system-managed")
run_config = RunConfiguration()
else:
run_config = RunConfiguration.load(path, run_configuration_name)
...

# default to path if source directory is missing.
if source_directory is None:
source_directory = path

```
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