ConvBERT: minor fixes for conversion script
Hi,

the conversion script for ConvBERT throws the following error message when using it:

```bash
Traceback (most recent call last):
  File "convert_convbert_original_tf1_checkpoint_to_pytorch.py", line 19, in <module>
    from ...utils import logging
ImportError: attempted relative import with no known parent package
```

I fixed that error, as well as using the correct name for the configuration file argument.

Additionally, I just found that the configuration files from the [YituTech](https://huggingface.co/YituTech) organization for ConvBERT from aren't correct, because they use:

```json
"model_type": "conv_bert",
```

instead of:

```json
"model_type": "convbert",
```

(This currently results in a `KeyError: 'conv_bert'` error).
I hate this forking/syncing stuff with GitHub 🙈

Preparing a clean PR now...