rootfs/etc: add pam file for sshd

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

userspace/rootfs_override/etc/pam.d/sshd

+#%PAM-1.0
+auth       required     /lib/security/pam_unix.so shadow nodelay
+account    required     /lib/security/pam_nologin.so
+account    required     /lib/security/pam_unix.so
+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