Fix permissions for /var/log/btmp (sshd requires it to be 0600)

The `logrotate::defaults::suse` class correctly uses 0600 as the create_mode for /var/log/btmp, but `logrotate::defaults::redhat` and `logrotate::defaults::debian` use 0660. That causes sshd to emit the following error:

     sshd[24031]: Excess permission or bad ownership on file /var/log/btmp

Relevant Redhat bug: https://bugzilla.redhat.com/show_bug.cgi?id=485553