System users should use the nologin shell to deny logging as those are
not interactive users but a lot of existing documentation incorreclty
uses 'su' or 'sudo su' to create a shell under the postgres user for
PostgreSQL management.
Revert back to using '/bin/bash' as the login shell.
See: https://src.fedoraproject.org/rpms/postgresql/pull-request/60
Fixes: 3646f8d Use sysusers configuration to setup user & group
