Fixes#1533 and #1534
Memsetting `turn_params.default_users_db` before reading conf file, not
after.
Because auth is read in first iteration so secret was wiped out.
# test plan
Add new test script that uses config file to setup turnserver instead of
cli arguments and confirm it works (fails without the change)
MacOS CI build started to fail in the PRs because when upgrading python
it was not possible to override one existing file (related to 2to3
package). Apparently this happens because MacOS runners some times have
python versions not installed/compatible with brew.
Example of the failure:
https://github.com/coturn/coturn/actions/runs/3850951324
The proposed workaround is taken from here:
https://github.com/actions/runner-images/issues/2322
