More docs, four new features, code updates

The galaxy page leaves it pretty unclear, what the package does. This readme improves that part.

Some variables have been flattened. With everything in a hardening dict, when one wants to change the username, all variables have to be copied. That is not required any more.

If one creates his user with a different role, he can just define hardened_user: false
This will prevent user creation

A user can define his shell now.

sshd only accepts very strong ciphers now. Can be deactivated. See the readme for more info.