Add sysctl to all images

This is needed to customize the default kernel keepalive timings, see
https://bugs.launchpad.net/oslo.messaging/+bug/856764/comments/19

Change-Id: I9a8ace4712c8a3b71f63b0bafe381e3bc6c707da
Partial-Bug: 1301431
Partial-Bug: 1385240
Partial-Bug: 1385234