#### Why I did it
To avoid the following error
```
Traceback (most recent call last):
File "/usr/local/bin/flush_unused_database", line 10, in <module>
if 'PONG' in output:
TypeError: a bytes-like object is required, not 'str'
```
`communicate` method returns the strings if streams were opened in text mode; otherwise, bytes.
In our case text arg in Popen is not true and that means that `communicate` return the bytes
#### How I did it
Set `text=True` to get strings instead of bytes
#### How to verify it
run `/usr/local/bin/flush_unused_database` inside database container
#### 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
* restoring each database with all data before warmboot and then flush unused data in each instance, following the multiDB warmboot design at https://github.com/Azure/SONiC/blob/master/doc/database/multi_database_instances.md
* restore needs to be done in database docker since we need to know the database_config.json in new version
* copy all data rdb file into each instance restoration location andthen flush unused database
* other logic is the same as before
* backing up database part is in another PR at sonic-utilities https://github.com/Azure/sonic-utilities/pull/1205, they depend on each other
