Do not process accounts with /sbin/nologin as login shell

The new logic of processing accounts with ssh keys has interfered with the ability to ssh as root on centos and RHEL images.  Those systems have operator accounts with a homdir of /root, which means that /root/.ssh/authorized_keys triggers processing of the operator account.  This can leave the /root/.ssh/authorized_keys file owned by user operator, and sshd will not root login because the ownership is wrong.  This pull request should skip processing of accounts with /sbin/nologin, which would resolve this issue.
Also consider adding a comment in the code to say why /sbin/nologin means to skip the account.