Fix python version (#7658)

#### Why I did it
To avoid the following logs 
```
Mar 15 15:52:04.599302 igk-dut-04 INFO database#/supervisord: flushdb /bin/bash: /usr/local/bin/flush_unused_database: /usr/bin/python: bad interpreter: No such file or directory
Mar 15 15:52:04.599947 igk-dut-04 INFO database#supervisord 2021-03-15 15:52:04,599 INFO exited: flushdb (exit status 126; not expected)
```

#### How I did it
Fix  shebang
#### How to verify it
Check the logs

dockers/docker-database/flush_unused_database

@@ -1,4 +1,4 @@
-#!/usr/bin/python
+#!/usr/bin/python3
 import swsssdk
 import redis
 import subprocess