Use CoreOS 1068.9.0

The differences between 1010.6.0 and 1068.9.0 are published by the
CoreOS team [1] and are noted below.

1068.9.0
========

Security Updates:

* libcurl 7.50.1 for CVE-2016-5419, CVE-2016-5420, CVE-2016-5421,
  CVE-2016-4802, CVE-2016-3739

1068.8.0
========

Security Updates:

* libpcre 8.38-r1 for CVE-2014-8964, CVE-2014-8964, CVE-2015-5073,
  CVE-2015-5073, CVE-2015-5073, CVE-2015-8380, CVE-2015-8381,
  CVE-2015-8383, CVE-2015-8384, CVE-2015-8385, CVE-2015-8386,
  CVE-2015-8387, CVE-2015-8388, CVE-2015-8389, CVE-2015-8390,
  CVE-2015-8391, CVE-2015-8392, CVE-2015-8393, CVE-2015-8394,
  CVE-2015-8395, CVE-2016-1283, CVE-2016-1283

Bug Fixes:

* Properly escape systemd specifiers (#1459)

1068.6.0
========

Bug Fixes:

* Fix parsing of the user database which caused systemd-sysusers to crash (#1394)
* Fix handling of certain unicode characters in bash (#1411)
* Fix issue when starting transient services (#1430)
* Include work-around for empty SSH host keys (#106)

Changes:

* Removed Kubernetes kubelet
  * The Stable channel has never contained a version which included the
    kubelet. If an instance was booted from the Beta or Alpha channels
    and then moved to the Stable channel, it will lose the kubelet when
    it updates to this release.
* Set group for /dev/kvm

Updates:

* coreos-metadata v0.4.1
* bash 4.3_p46

[1] https://coreos.com/releases/

Change-Id: I00c2114aeea9d4c1b8e38f5786158f248875970d

imagebuild/coreos/version.txt

@@ -1,7 +1,7 @@
-COREOS_BUILD=1010
-COREOS_BRANCH=6
+COREOS_BUILD=1068
+COREOS_BRANCH=9
 COREOS_PATCH=0
-COREOS_VERSION=1010.6.0
-COREOS_VERSION_ID=1010.6.0
-COREOS_BUILD_ID="2016-06-28-0910"
-COREOS_SDK_VERSION=1010.5.0
+COREOS_VERSION=1068.9.0
+COREOS_VERSION_ID=1068.9.0
+COREOS_BUILD_ID="2016-08-05-0455"
+COREOS_SDK_VERSION=1068.8.0