postfix_null: Use fixed amount of rounds for password_hash

As it turns out the value for this filters "rounds" parameter strongly
differs depending on the installed python crypto backend, since
python-crypt uses 5000 rounds while python-passlib uses 656000 rounds
set a default parameter according to ansible documentation.

As really high values for "rounds" lead to some login timeouts it makes
sense for us to use a fixed value for this parameter. In this case 5000
have been chosen as this value reflects the defaults from python-crypt
aswell as /etc/login.defs in the shadow package.

Link: https://github.com/ansible/ansible/pull/77963/files
Related-to: #250


Signed-off-by: Christian Heusel <christian@heusel.eu>