Add discard old password support

[sov-msaeed]

Hi @petoju

As described here #176 I have tried to add support for mysql discard password. I have tested the provider locally to one of my RDS instances

• Creating user with password1
• changing the password to password2 with retaining the old password.
• Then enabling discard the old password property, which deleted the old password so only password2 works ✅
  I also tested changing the password with enabling the 2 property retain_old_passowrd and discard_old_password. incase a user does that by mistake, the discard_old_password is doing nth so the new password will be created and the old password will be retained.

What I am not so sure about is the test cases. I tried to add the functions and use cases which i described above but maybe i am missing something.

Happy to get your feedback.

[petoju]

Thanks! Everything was in order, so I merged it.
This was released it in provider v3.0.70.