rootfs/etc/pam.d: add libpam-krb5

Signed-off-by: Adam Wujek <adam.wujek@cern.ch>

userspace/rootfs_override/etc/pam.d/sshd

 #%PAM-1.0
+auth       sufficient   /lib/security/pam_krb5.so minimum_uid=1000
 auth       required     /lib/security/pam_unix.so shadow nodelay
 account    required     /lib/security/pam_nologin.so
 account    required     /lib/security/pam_unix.so
+account    required     /lib/security/pam_krb5.so minimum_uid=1000
+# don't support changing kerberos password
 password   required     /lib/security/pam_unix.so shadow nullok use_authtok
 session    required     /lib/security/pam_unix.so
 session    required     /lib/security/pam_limits.so
+session    required     /lib/security/pam_krb5.so minimum_uid=1000