Add __repr__ for Executors (#13753)
Add __repr__ for Executors (#13753)

Before:

```python
>>> from airflow.executors.local_executor import LocalExecutor
>>> LocalExecutor()
<airflow.executors.local_executor.LocalExecutor object at 0x7f49b47f8d68>
```

After:

```python
>>> from airflow.executors.local_executor import LocalExecutor
>>> LocalExecutor()
LocalExecutor(parallelism=32)
```
apache/airflow,sekikn/incubator-airflow,danielvdende/incubator-airflow,sekikn/incubator-airflow,bolkedebruin/airflow,nathanielvarona/airflow,danielvdende/incubator-airflow,danielvdende/incubator-airflow,nathanielvarona/airflow,apache/incubator-airflow,cfei18/incubator-airflow,cfei18/incubator-airflow,danielvdende/incubator-airflow,lyft/incubator-airflow,dhuang/incubator-airflow,cfei18/incubator-airflow,mistercrunch/airflow,apache/airflow,bolkedebruin/airflow,Acehaidrey/incubator-airflow,nathanielvarona/airflow,apache/airflow,Acehaidrey/incubator-airflow,lyft/incubator-airflow,nathanielvarona/airflow,lyft/incubator-airflow,nathanielvarona/airflow,apache/incubator-airflow,danielvdende/incubator-airflow,apache/airflow,apache/incubator-airflow,apache/airflow,Acehaidrey/incubator-airflow,cfei18/incubator-airflow,dhuang/incubator-airflow,mistercrunch/airflow,sekikn/incubator-airflow,cfei18/incubator-airflow,mistercrunch/airflow,Acehaidrey/incubator-airflow,apache/airflow,sekikn/incubator-airflow,bolkedebruin/airflow,dhuang/incubator-airflow,mistercrunch/airflow,Acehaidrey/incubator-airflow,Acehaidrey/incubator-airflow,bolkedebruin/airflow,danielvdende/incubator-airflow,dhuang/incubator-airflow,lyft/incubator-airflow,nathanielvarona/airflow,apache/incubator-airflow,bolkedebruin/airflow,cfei18/incubator-airflow