ensure sysctl.d exists (it does not by default on CentOS)

add a block to create /etc/sysctl.d, which is created by default
on ubuntu, but not centOS (though as of centOS 6.2, it is supported)

Change-Id: Id8011d7ef91cc13182f45adc201a3c66c8e891cd
Closes-Bug: #1266914
Implements: blueprint centos-rhel-for-common